Well,the next logical step would be to check for 12 volts at the inverter.
BUT,some things you can check-
1 all the fuses and circuit breakers
2 you might have a separate "changeover/transfer switch/module, and it may have a switch on it
3 My best guess--- some Forest River products have a battery disconnect switch, that only disconnects the batteries from the inverter. Usually located in the battery compartment. Dealers turn them off because the inverter can discharge the batteries when sitting unused for a while.
Good Luck.