Ford has charging solenoid mounted from the factory, but the program how to activate it is tricky.
Still remember one winter night, when we had low battery warning at 4 AM.
I have remote start on Ford, so started the engine to charge camper battery, but turns out remote start is not activating the charging solenoid.
Had to go to truck and put the key in.
Why other brands don't have solenoids factory mounted?
My other idea is to use truck dual batteries instead of adding more batteries to TC.
I hesitated to do it so far scarred with possibility of running truck batteries too low for engine start, but this season I bought the 1000W inverter generator who with its 20lb of weight is easy to carry as backup even when I don't carry big generator for AC.