I agree you may have a blown fuse or a failed converter.. With the exception of the microwave and SOME air conditioners and the Television nearly everything in an RV needs `12 volts. MOst A/C's (Wall mount T-stats or IR Remote T-controls need 12 volts.. Water heater. fridge, frunace. lights. fans evcen some TV's are all 12 volt. or need 12 volt for control power. No 12 V they shut off.
Also check the 120 volt breakers Look for one marked CONV (If they marked it consider yourself lucky)
And with the rig UNPLUGGED operating by 12 volt and/or flashlight check the wires in the breaker box for loose screws.. Many have 'em.