Virtually all RVs are set up to charge the house battery from the engine alternator. Sounds like the system was modified and eventually left disconnected. Commonly there is an isolator to allow charging while the vehicle is running and prevent discharge of the chassis start battery when parked. Need to reconnect or create a new connection. Second battery is a must and might not even be enough.
Charging through the alternator is generally slow due to limited power and long thin wires. A DC-DC charger can really help. Can still take 12+ hours to get a low battery fully charged.
Consider some solar in the future plans.
The 12 Volt Side of Life
