Use candles, lanterns and LED flashlights. Avoid any inside 12v lighting.
Only use the batteries to power the control boards on the fridge and
water heater. IF you have to use water from your tank of course you'll
use battery for that. Make sure batteries are fully charged when you
IMO plugging the trailer into the bumper socket on your truck won't charge
much due to the long distance from engine to trailer battery.
Jumper cables from truck to trailer batterys as mentioned will work somewhat.
If I were camping in a "quiet" no generator campground and someone was
running their truck engine to charge batteries as a way to get around the
quiet rules I would be upset.... I'm just sayin..... they have the quiet rule
for a reason.
I still think if you're super careful you can do it on two decent 12v batteries.
Please come back after your trip and tell us how it worked out!!