Some modern alternators will not start up if there is less than ~10.5V on the battery. This is a safety factor, they say, to make sure you have a professional find out why your battery died. Don't know if your jeep has that problem, one hopes Jeep has a better attitude.
But don't bother with a battery ISOLATOR, that's a set of diodes and that means voltage drops and the alternator voltage sense lead can never be connected properly. Obsolete technology.
The simplest way to do this is with a battery COMBINER, i.e. Yandina (aka West Marine) or Hellroarer or Blue Seas. Also sold as an "echo charger". These all work similarly, when they see 13.3V (some 13.
at your primary battery, they connect the secondary battery and allow the normal alternator to charge both batteries in parallel.
That's not a perfect way to charge two batteries, but in reality it works out very well. It is simple, cheap, reliable. The only catch is that both batteries should be of the same type, i.e. both wet acid or both AGM or whatever your choice is, designed to charge by a car alternator at 13.8-14.4 volts. Your deep cycle battery will last longer if you also arrange some other means for better charging, occassionally using a 3-4 stage charger on it or bringing it up to a real 100% charge once in a while. A car alternator never really does that. Be generous, use something like a 4AWG cable for the second battery, and instead of grounding it to the jeep, you may want to run a ground cable back to the same grounding point that the primary battery uses. (Your choice, that's debateable.)
Also use a suitable FUSE on the secondary battery, normal breakers arc over and weld around 3000W and a battery can aesily do that in a dead short. You want a proper ignition-protected fuse at the second battery, and a disconnect switch or other protection where the primary battery cable goes to the combiner. Among other things, so you can disconnect the secondary and ensure "no surprises" if you dop it off for service someplace.
After 8 years of running this way I finally had to replace the secondary battery. That's not bad battery life, it works.