There may be a switch near the front door that is labeled battery disconnect or something similar. On my rig this controls a solenoid in the back electrical compartment, the solenoid shuts off all 12 volt to the house.
If you have a switch flip in on/off and see if that fixes it.
If not and you have the solenoid it may be bad, they are known to fail and you can bypass if by just connecting the 2 large cables together and tape up the small wire.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi