98 monaco executive

ok here we go. got to check out friends monaco electric problems. cleaned all grounds i could find, all systems seem ok,dash voltage guage still shows 10 volts. battery tested at 13.5 volts with motor running but owner says if you use any 12 volt while driving batteries go dead, so they run generator while moving. do any of you know if they run the power thru volt gauge like mopar use to do? help please i am lost.