If you are having troubles with the coach 12 volt system only , then perhaps the " Gulfstream " owners forum would be the place to ask. Other owners with your brand , may be sure what kind of battery control center your manufacturer used and how to check and repair it.
Most systems have a master fuse or 12v circuit breaker , and it should be within 3 feet of the coach batteries.
Then there is the possibility of a defective , disconnect relay , or salesman switch.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.