I can get four oe five days easy running lites, gas fridge, gas hot water, and water pump. As long as I don't use furnace, no problem.
2010 Ford Expedition TV
2010 Outback 230RS Toybox, 5390# UVW, 6800# Loaded Not yet camped in Hawaii, 2 Canada Provinces, & 2 Territories I can't be lost because I don't care where this lovely road is going
On a single battery and if you lost the power grid for what ever reason and it got really cold at night causing you to run the heater, the battery will be almost drained by morning. If you have two batteries you should be able to go 2-3 days.
Ref your question about hooking up the tow vehicle to charge the batteries. This is a very slow process and it will take from 4-6 hours depending on how low the batteries are. Plus, it will hit your gas tank also.
I have two batteries in parallel and if there is any chance I will be off the power grid I take along the Honda 2000 and a automotive battery charger.
2001 2500 Silverado LS 4x4 6.0, 4.10
2007 Jayco Eagle 288RLS
Cocktails for 6, Dinner for 4, Sleeps 2
End of dirt road, no signs in sight, sun going down: I am not lost, just getting ready to go camping.
So, how long you figure a 12v battery will last running the refer on gas?
Depends on fridge model. With 10 Ah daily draw and 90 Ah battery - 4 or 5 days to 50% discharge. Maybe - 6, with Peukert correction for small loads. There were numbers reported up to 15 Ah for fridge alone. Plus, another 5-7 Ah a night for radio standby, serving no other purpose but keeping a clock time and a few favorite UHF stations presets.
Doesn't look like much, but those 17-20 Ah is more than LED lights will draw - you don't really sit inside reading a book on a bright sunny day. The OP's concern about "lights and pump" is a little misguided, IMO.