There are two common configurations for Motor Homes
1: Standard isolator: In this system the engine's alternator charges both the chassis and the house (Aux) batteries when the engine is running.. Now depending on the isolator "logic" if the chassis battery is defective it may never engage the solenoid, but you would have starting issues if that were the case... Likewise a defective alternator or a loose drive belt on the alternator can cause issues.
In this system shore or generator power ONLY charges the house, not the chassis.
2: BIRD (BI-directionsl Relay Device) this one lets the tngine charge both sets of batteries, Same as 1.. but it also lets the house converter (Shore/Generator) charge the chassis (main) battery
If the alternator is charging the chassis battety, but not the house, your isolator is defective. OR there is a bad conneciton somewhere along it's wires.
Home is where I park it!