Start with the most simple things first. Check your house battery connections. Check the series cable that connects the two 6 volt house batteries together. (if you have two house batteries) Sounds like you have a bad 12 volt connection somewhere. Don't forget the ground connections. A multi-meter is your friend.
When you start the engine, the engine alternator provides 12 volts to the house systems which can bypass the house batteries depending on how its wired. Look for non-factory electrical work. At the age of your coach, you could have anything, as altered by previous owners/electricians.
Chum lee
