I've got a setup kinda like what you're talking about. I purchased a battery isolator from JC Whitney (www.jcwhitney.com
). It takes the output from the alternator and has two output terminals on it. One for the main starting battery, and another for the auxiliary battery. The isolator keeps the batterys from discharging to one another.
I purchased my auxiliary battery from The Alternative Energy Store (www.altenergystore.com
). It's a nice MK 12v gel cell that weighs about 25 pounds. I got a nice battery box from a marine store I found on the web and then built a small shelf in the back of my Isuzu Rodeo for this box to sit on right above the wheel well so I wouldn't lose any space. I got a Coleman power invertor and used some velcro to mount it on the side of the shelf.
I then ran power cables to underneath the drivers seat from the auxiliary battery into a Rig Runner 4005 from West Mountain radio that I mounted on a board. This provides me with a good DC power outlet for my radios, etc. I've also built a 12v-5v DC-DC convertor and mounted it on that same board. I use this to power a GPS board I've interfaced with my TM-D700A, as well as a MAX232 level converter.
Hope this helps. Good luck.