You should have 12 volt power for lights, vent fan, furnace etc. unless battery is dead or disconnected.
You should have 120 volts for air conditioner, microwave, converter which keeps batteries charged when plugged in or running generator which has it's own breaker.
As mentioned, check breakers, battery disconnect switch and battery (water level in each cell, cable connections).
Let us know what you did to solve please.
2017 Forester 3051S