Does your RV have a ladder on the back?  That is the general location most hams install their HF antennas.  You can put a bracket up top with a base mount and remove and install your antenna ( whatever type you use )  when you park;  or you can put a more elaborate mount on top of the ladder that allows you to hinge the HF antenna down onto the roof of the RV for travel... a bit more complicated as you need to install some roof stand off supports for the antenna to lay in to protect your roof .  I started off with simple Hustler mobile HF antenna which required me to change loading coils manually when changing bands,  and I had to manually screw it on the base and remove it for travel.  But it worked well.   I now use a 23' vertical made of aluminum telescopic tubes  ( from Texas Towers,  or  DX Engineering )  and have a remote tuner also mounted at the base of the vertical on the ladder.   Whatever you use on the ladder you should make sure you have a good counterpoise.  In my case the ladder goes down to the rear bumper area on my RV and I simply run a ground cable to the chassis underneath the RV.  If you can't do this then you should probably simply run a wire counterpoise out on the ground ....  one or two at minimum.   I ran my coax underneath the chassis and up through the floor near where my station is mounted in my motor home.... it might be slightly easier with the 5th wheel to run your coax.  I recommend if underneath the RV you also put some split loom around the coax.   Don't forget you'll need 12VDC  for an external remote tuner if you mount one of those at the base of a vertical.  In my case I used a couple MFJ Bias Tees and run the 12VDC in the coax until it gets to the tuner.

  If you do NOT have a ladder on your RV,  you may want to consider mounting your antenna on some exterior bracket available.... perhaps part of the awning extensions over your slide outs?  I have seen excellent HF vertical mounts on the side walls of RVs  using marine type exterior antenna mounting brackets.   Some go the whole with with the marine products and use the Shakespeare HF antenna too.   These are generally tuned with external antenna tuners... sometimes mounted outside the RV,  and sometimes the tuner is mounted inside and a simple short wire connects the tuner to the base of the antenna.

   Just a few ideas of what I have seen or implemented myself.

  Ed   K7AAT

  Read page 35 of your manual.

  Ed   K7AAT

  It seems to me that the critical lengths of your balanced feed line would depend on the length of your wire antenna ...  which you did not specify.

   Ed   K7AAT

   Just to emphasize,  in case you glossed over it .... go up and re-read the post from W4DRR.  He gave you the most accurate and useful answer to your question.

   Ed   K7AAT

   Have you set the IC-746Pro for DATA mode,  which is required for the 6 pin data jack use?  Push the SSB button and HOLD it for a moment... a small  'D'  will show up to the left of the USB at the top of the display screen.... and you are now in DATA mode.  The radio should transmit now when the SignaLink sends the PTT signal.

   Ed   K7AAT

  While I can not say with 100% certainty,  I'd strongly suggest that the FT-1802 does NOT have a memory backup battery.  Every late model radio that I know of in the past several years was designed with non-volatile memory;  no battery needed.  I'd be real surprised if it was not the same.   No where in the operations manual,  or the Technical Supplement manual can I find any reference to a memory battery.  While I may have missed it,  I could not find one in the schematic, either.   I think the original poster's problem may be elsewhere in the radio.

  Ed   K7AAT

  You really need to describe your window.  There are multiple kinds.... the normal kind as I view it that slide closed in tracks,  either vertically,  or horizontally.  MFH makes a great product for that kind... or its easy to built your own.

   If,  on the other hand,  you are speaking of the "crank closed"  kind that is hinged on one side and closes while you crank the handle,  that could be a little more difficult.  Without spending more time for thought on this I would first suggest you look into various kinds of stick on weather stripping you could attach to the edge of the window...  keeping out bugs and air,  but with a small space just to let in your coax.

   Regarding drilling cinder blocks.... I believe you can get foot long drill bits .... certainly metal drills... and probably masonry bits that long too ... and if you drill through the mortar instead of the cinder block it should relatively easier .. and surly easier to patch when you no longer need the hole.    Good luck in any case.

  Ed   K7AAT

  Its probably still sitting on a warehouse shelf at a ham radio dealer's business,

  Ed   K7AAT

  I don't know where you get the 246 number.  I is not related at all to the standard 468/f formula for half wave,  or 234 for quarter wave.   I assume you are ALSO figuring in the velocity factor of your cable before you make the cut ?

  Ed   K7AAT

   As others have stated, a good answer depends on the details,  which are not provided.   In a broad sense,  transmitting into NO load should be worse than transmitting into a grounded cable which IS a load of some sort. 

   Do you realize that many antenna designs ARE short circuits, by the way?   Some Gamma matches are short circuit to DC.   So is a folded dipole and other such related antennas.  A short circuit does not necessarily mean a bad load.

    Ed  K7AAT

  Many still consider the Yaesu FT-100 a nice little compact HF radio.    If I were in the market for an inexpensive fixer-upper,  I'd make you an offer now.   I do not consider a stuck squelch  (  not "squelsh" )  to be a serious issue to address and I suspect there are plenty of other hams who also would consider it an easy fixer-upper.  Don't list it as a "parts"  radio;  list it as having that one problem and lower your price accordingly.  You will have no problem selling it as is.

   .... if your skills are up to it,  you could try repairs yourself.... or perhaps find a skilled friend to look at it?...

   Ed   K7AAT

  James,   your post leaves OUT too much information for anyone to really give you a good personal assessment of what you should look at.  You don't include budget limits,  what parts of the amateur spectrum you wish to operate ( HF? VHF? UHF? ) ,  modes you might prefer,  and other such preferences.   Given such little information,  I would be prone simply to suggest you get an Icom 706MKIIG  or and Icom IC-7000.... both of which would allow you to operate on all the popular bands and modes,  and they would function nicely as both mobile and base unit  ( base would require a 12VDC power supply or battery and charger).    Yaesu make a nice little unit too.. the FT-857,  but I prefer the Icom products for usability.   Wire antennas for HF base stations are cheap.   You can start with dipoles.... all tied to a single 50 ohm coaxial cable and that will give you resonant antenna operation on several bands.   Good luck.

   Ed  K7AAT

  There are only four 10M repeater channels in the U.S.,  all so noted on any HF 10M band plan chart.   Standard offset is 100KHz low.

  Ed   K7AAT

Memory is only partly serving me... but you are right on the C or D cell battery.  The other battery is a weird mercury battery... I can not remember if it was for 12V  or some other voltage.   I also somewhat recall a later version that took either one or two standard 9 volt batteries.  Unfortunately I sold my manual for that unit a few years ago.    I'd be surprised if you can't get this information with further research,  or from other respondent here.   Good luck.

  Ed   K7AAT

Previous Poster: "Jim, please explain what you mean by the statement "the amp is acting a little weird."

   Jim did give a more detailed explanation of his meter readings, etc,  which you apparently ignored.

    Previous Poster:  "If you want to play with firearms, you need a FOID card prove you are compentent to do so. If you want to play with high power, you should also need a 'seperate' license to be allowed to run, say anything over 200W. "

    Most States do not require any license or "FOID" as you call it to  "play"  with firearms.    Additionally,  no state requires a spelling test to post to Internet forums,  which is good news for you.

    Ed   K7AAT