[G.R.]

Can't help you with the car charging part probably could be done though at a sacrifice of one of the other batterys.
If I have this right you have 2 trailer batteries? I assume the "travel trailer" battery is the one that powers the interior lights/outlets? If so it could be changed out to a deep-cycle type, it would recieve it's charge when the trailer is plugged into the tow vechicle and the tow vechicle is running, otherwise it would stand alone and power the trailer not drawing off the vechicle battery bank.

Not sure what you mean by a "breakaway" battery? I'm very familiar with using battery power and setting them up on sailboats--in a boat you have a battery switch, simple setups uses a 1-both-2- off system rotary switch, 1 was used for the "start" battery in that position only the engine start battery was charged, 2 was for the "house bank" this position only charged the "house bank" and both charged both banks.

I for years used 6v batteries wired in series as a house bank up to about 2400 amp hours. And a heavy duty 12v start bettery. Then I switched out to 12v Rolls deep cycles for the house bank. Used these for almost 8 years until we sold the boat, 4-800 amp hour batteries. We often anchored for a week at a time, running refrigeration/freezer, lights, radios (vhf and ssb), and stereo, fans, inverter for small tv/vhs, before having to turn on the engine to recharge the "house bank".

If I were to go for a deep-cycle I'd seriously consider using the Rolls again. They'll absorb a super deep discharge and re-charge very fast--you do need a large amp alternator though.

Hope this helps