If you want to test the wiring, the fuel tank switching unit on the frame rail(which switches the sending unit wires) and the fuel gauge in the dash, put the fuel switch to the proper tank, take the wires off the sending unit, and with the key in run, engine off, ground the sending unit wire, and then take it off ground. Each time you do this, the gauge should swing full scale back and forth. Do this for both sending units, and that will tell you if everything is working, and the sending units are bad. I would not think it would be uncommon for both of them to be bad, since it's very common for them to fail, and most people will keep driving it with one failed unit, and the next unit usually will fail within the next year or so.
The tanks are very hard to pull down. You have to use a floor jack, expecially since you can't get most of the fuel out. That fuel is heavy, and you need to two people even with the floor jack. The tank will suddenly tilt one way, all the fuel will slosh that way, and there it goes out of control. Once you do get it out, you can pour all the fuel out and it's much easier then to get it back in.
The bed removal thing has it's own problems as was mentioned. The main one being the bolts being rusted. On a couple of them I had to take a 4 inch grinder and grind the heads off in the bed, and then lift the bed off. Then you have to take a sawzall and cut the rest of the bolt off to get it out of the frame.
But taking the bed off, you can go ahead and clean the frame up and paint it, and no matter how you pull the tanks, I would take this opportunity to redo the tank venting. I plan on doing mine, since filling it up with fuel is a nightmare. One of the posters was describing how he threaded a 1/2 pipe fitting into the rubber of the original vent in the tank, I think I am going that route.