Lights inside a Motor home are normally 12 volt DC devices, I need to ask what else is not working.. Suspect list includes slides and water pump, the fridge and water heater and furnace and fans.. All use 12 volt either for operational power or control power.. Likewise if the air conditioners have remote thermostats (by remote I mean wall mounted OR wireless remote controls) then they use 12 volts for control as well.
Everything not working is 12 volts.
Suspect#1: Your battery disconnect (Salesman) Switch..
Suspect #2: Dead house battery
Suspet #3: Dead or disconnected converter (This can be due to a tripped breaker or in a very few RV's tripped GFCI)
and if only SOME of your 12 volt stuff has failed....
Suspect #1 blown fuse #2 bad junction connection or wire.