I think you could go several days by being conservative with the power. Be sure to check if the anti-condensate switch on the refrigerator is turned off.
This goes against mainstream advice so take it as you will:
1. Most people advise against discharging batteries more than 50%-as they'll take a hit on expected life. While this is true, it is relevant for those who charge and discharge their batteries very frequently. Charts I've read indicate about 1000 recharge cycles at 50% discharge vs. about 250 cycles at 90% discharge. Frankly if needed, I'd use one of those 250 cycles-the batteries are not that expensive. Keeping the batteries fully charged when not in use will do more for life IMO. Link: DEEP CYCLE BATTERY FREQUENTLY ASKED QUESTIONS
2. Are you planning on driving the tow vehicle daily to sightsee? If so why not share a little of the battery duty with the tow vehicle's batteries? You have two big ones in the diesel Excursion, that really won't be working hard to start the engine in the summer. Since most Fords have a relay which cuts power to the 7 pin engine off, I'd make up a fused jumper cable long enough to reach from front of tow vehicle (backed in for access to emergency jump start if needed) to trailer batteries (leave it hooked up during evening hours while you're awake). Again, a few extra discharge cycles may shorten the life of the batteries a little. But this isn't something you plan on doing often.