Your isolator is not working.. This consists of two parts on MOST RV's one on a very few. I will describe both.
Most common.. A Solenoid (Also called a Relay or Contactor but in this use Solenoid is the most common name) and a controller for it.
Depending on the controller, it may be a one-way (Engine charges house) or it may be a bi-directional like min (Both charge).
The simple ones may or may not have a delay and sense the ingnition lead.
The more complex ones measure voltage and when the see 13+ they engage
To test you need to find the isolator relay and measure the voltage on the SMALL screws. (the big ones are battery leads) you should see voltage within one minute of starting the coach.. And that voltage may appear and dissappear depending on teh system.. But I suspect it will be constant.
