Are you sure it's the door, and not the bunk board against the roof of the cab?
Quite frequently, squeaks develop here because the screws they use to attach the bunk board to the cab work loose or rust out. You can get at them by removing the trim and pulling the headliner material away. (On my RV, at least, it's also necessary to unscrew the visors and their clips, etc. to get the headliner out of the way...in any case, it should be pretty obvious how it works with a bit of examination.)
I found this exact problem, and have cured it--I hope for good--by adding a dozen countersunk carriage bolts clear through the bunk board and putting flat washers and nylock nuts on the underside to secure them together much more sturdily than the little screws that were there. I have an even bigger grin on my face when driving down the road now than I did previously.
Sometimes there are cabinets etc. that can also squeak some as things move because they're not tightened down properly, but the bunk board is the more common problem.