Some of us are blessed with An intelletec Battery Control System featuring BIRD technology (Bidirectioal Isolator Relay Device) and this means our house system charges our chassis when parked, as well as the other way around when driving, This also means if ye old alternator ever fails to alternate.. I can get Mr. Onan to do the job with a bit of help from the Progressive Dynamics 9180.
Older and even some current models do not have this, they have one-way isolators, this means Engine charges house but.. not the other way around.
Solution is a device called the Trick-l-start
Or one called the Echo-Charger (more expensive)
Or, I heard there is now a 3rd model out.