12V 200A relays are pretty easy to find (I wouldn’t get one from alibaba tho), but that much current requires quite beefy wiring as well. Personally I’d review options to place auxiliary batteries in parallel since that would simplify wiring and the whole system a bit, but as I don’t know how your camper is built it can be tricky or you need to sacrifice storage space somewhere.

And as you’re placing a battery at the same space where people stay be careful with hydrogen. Charging lead acid battery produces hydrogen and in the worst case scenario, specially if you have a gas stove be dangerous if not lethal.