Well, if it is 110 only, you are kinda stuck.
1) pack it with ice in ziplock bags so the heater does not run as often and the water does not run all over the place
2) More batteries (deep discharge) not dual purpose (or
starting batteries). The POS one that you have now is most likely 60 AH or less. A pair of 100 AH batteries would do the trick.
Trojan Battery Company
3) Solar panels on the roof to assist keeping the battery bank up.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW