Welcome to the forums.... looks like it could be test light/fault tracing time. In my '76 Argosy the Univolt had already been replaced (which you will probably end up doing sooner rather than later) but they decided to use the Univolt fuse block. Caused me no end of problems as the fuses tested good and there was power through them but the connections on the back side of the panel were loose and it changed as the trailer moved. The system is fairly simple but almost 50 years old. Start at the battery and trace your powers and grounds. In the 12v
system white wires are grounds and may just go to the body.
120v wise many have trouble with a 30 amp "RV only" plug and sock looking a lot like a 220 plug and socket, NO interchange. Agree that humming Univolt could be a sign of normal operation, get the 12v
stuff working first. Would suggest a new converter/charger (many good options out there, I like the Boondocker units) and a Blu Sea 12v
fuse panel with positive and negative busses built in like model #5025. Hope this helps....
Good luck, Mark D