Is the battery charging while plugged in even though the monitor panel does not show "C"? Put a voltage meter to the coach battery after you have been plugged in for at least 12 hours and see what it reads. If it is charging then there may be a problem with the monitor panel or the connection to the panel from the battery.
It is not really clear though about what you are saying about what happens when - when on battery only - battery switch on - do all the monitor leds that are supposed to light light up - fresh water, grey tank, black tank, propane, and Battery?
The manual for the 1991 is sparse - as were most of the early manuals. No photos and barely a description. Here is a link to the manual.
http://www.roadtrek.com/_Uploads/manuals/Roadtrek_Owners_Manual_1991_(01)_All_Models.PDF