If you're wanting to sell the Dometic, I may be interested. Contact me by a PM so we can talk. Stratford isn't all that far from me...
Secondly, interior lights in older trailers not working is a fairly common problem. I had it bad. Things to do: get a couple of boxes of fresh bulbs. While that's likely not the ONLY problem, for SURE there are a bunch of bad ones in there.
Second, if the repair guy is right and your Univolt is OK (mine was borderline, just worked enough to be irritating) then next thing to check (after fuses) is the connections in each light fixture. They are only connected with wire nuts (Marrettes) and are not soldered, and so they very often corrode. Take them apart and clean them up. make sure the fuse clip is clean and not corroded.
Also look for actual missed connections. One of my dome lights had the supply wire going outside, just under the vent cap, and its connection had fallen apart. Re-did the wire nut, and it worked (with four burnt out blubs and only two that lit).
If this sounds too confusing, pass it to your neighbour/electrican and get him to start poking around, it's not rocket surgery, as the joke goes.
Also on mine, the wires (for two of the circuits) that connected to the Univolt were poorly connected to start, or had become corroded with time. I loosened the fat grub screw for each of them on the Univolt, cut off the end till I got to a fresh part of the wire, and re-connected. That brought the bathroom lights on for the first time.
Ultimately though, you'll want to replace the Univolt. Almost all of them buzz when they work, and by now, none of them seem to work too well. A somewhat bitter pill to swallow, but it's an investment worth making.
Last thing: if you decide to do this yourself, shut off (disconnect) the shore power before fiddling with the Univolt. 12V
isn't much, but it's still no fun getting zapped