I would think that Jayco would use BOTH batteries to start the generator, if not I would rewire it to use both batteries. Why.. if you have 2 12VDC batteries fully charged at 12.6Volts and you only use one to start the generator, you may have dropped that fully charged battery down to 12.4 or 12.3 volts and then you throw the switch to reconnect them and now the other battery that was 12.6 is no longer going to be 12.6 as it will drop in voltage to balance somewhere between the 12.6 and the 12.4. Continuing this process will shorten the life of both batteries. It is better to leave both batteries connected and watch the battery(s) voltage if it does not start on the first or second crank.
If you have a piece of land that you will be keeping your TT at, you may want to look into SOLAR ($$$) and purchasing some TRUE 6 VDC deep cycle batteries. I always recommend that when you first start dry camping or adding solar, keep the battery(s) that came with the TT and practice on them, and when you kill them and learn how to live on SOLAR you will be ready for the more expensive True Deep Cycle batteries.
To give you an idea, since I installed SOLAR, I have had the TT's battery charge controller turned off. The sun takes good care of the batteries. They are fully charged by 2PM on sunny days and 4ish on cloudy days. 200 watts would be the very minimum panel output you would want. If your TT is parked in a lot of shade and you have them mounted on the roof, you will need at least 400 watts. Buy a system that has an MPPT controller which will increase the Amps.
We have an "RVing with SOLAR
" social group here on the forum. There is a lot of battery, solar charge controller (PWM vs MPPT) and installation information.
2013 Jayco Eagle 284BHS
250Watt Grape Solar Panel, MorningStar MPPT 60 Charge Controller
1500 Watt Ramsond PSI, 2 Trojan T145 Batteries (260Ah)
2 - AirSight Wireless IP Cameras (used as rear view cameras)
EnGenius WI-FI extender, D-Link wireless (n) modem
MagicJack Internet Phone
2012 Ford F150XLT, EcoBoost w/3.73
157" Wheel base, HD Towing Package
Our Solar Album http://www.jaycoowners.com/album.php?albumid=329