Battery charger upgrade and purpose of solinoid

I don't know about truck campers, but if the battery charging circuits are similar to other RV types, there are plenty of alternatives. One limitation might be available space.

The solenoid is basically a high current relay. Its function in my MH is to connect the house battery to the chassis battery when the ignition is turned on. That way the house battery gets charged by the alternator. I'm surprised this isn't standard in a truck camper.