[willy]

On the fuel tank you can go a few ways, one drain into 55 gallon drum, and put in fresh fuel, get a local recommendation for your area about a tank cleaning agent and put it in with a few gallons of fresh gas hook that bad boy up and take it for an extended ride on the trailer stopping and going so the cleaner get sloshed around and drain again into separate container.
If it is real cruddy what I would do is find your local tank cleaner facility and pay them to give it a full cleaning, they will drain, use special solution under pressure and clean, drain again and filter your old gas and put back in. Buy a few fuel filters and be prepared to change them for the first few times you take it out.
Electrcal wise it sounds like a follow that line time, trace the runs, label them with marked tape pull all fuses and inspect, often it is in line fuses that get you. They are not very moisture resistant. Most accessories go to a common ground connection block under the console and often it is the ground wire that is corroded or disconnected and causing a problem. I hate wiring, always was a pain in the arse to me.
There is a good how to post concerning doing the tank covers over on this site, fellow did a nice job. Did not look to bad for a do it yourselfer and a good first fiberglass project to do. It is removed and open and easy to see.