Charging motorhome batteries with a solar trickle charger

The simplest solution would probably be to have a separate panel/trickle charger for the house battery. The two batteries are isolated when the engine isn't running, disconnect switch on or off. (That might technically be a bit of a simplification depending on the exact setup and kind of isolator involved, but it's conceptually close enough for what you're asking regardless.)

I suppose another perfectly reasonable solution is to just not cover the RV for the winter. I don't cover mine.