This one may be difficult to understand and on phone a pain....here we go.
Your rig should have battery disconnects that turn off the batteries.
Many are magnetic latching meaning they get a pulse and set then a different pulse to unset.
On a friends Fleetwood product which yours is these are powered by the cabin battery.
A bad cabin battery will not have enough voltage for the selenoids or relays to operate.
Try this and you may need a helper.
Locate your battery switches..
Locate your boost or emergency start switch.
This cross connects the start and cabin battery.
On our friends unit battery switches above entry side door and boost switch on dash so 2 folks needed.
Have helper push and hold boost switch then try turning batteries on.
Also under hood may be a box with fuses and cut breakers and some require a push of a button to reset.
Others may have similar unit that can help you navigate these suggestions.
A voltmeter will help and harbor freight has them cheap.
Tony & Lori
1989 Country Coach Savannah SE