Not sure what brand of campers you guys have but why not just put (2) LifePo4 cell battery’s in the same battery storage compartment that is allready there?
I have a 2019 AF 990 and a couple of years ago replaced my original lead/acid group 29 battery’s with some 280ahr LifePo4 cells giving me 560ahrs total. I was able to fit in a 200a BMS as well as (2) 150a rated terminal blocks for all my branch wiring.
I was also able to install a 1/4” foam box completely around the battery’s as well as having a top cover to provide thermal protection from the elements.
I even placed a temp sensor inside the battery’s compartment as well to a display that I can monitor the actual temp inside and last but not least cut in a small vent cover to allow heat and a small circ fan to circulate ambient warmer cabin air into the battery’s as well box if needed.
Never have needed it and have camped in snow conditions a few times.