If your problem is that the hinge screws are loose because the wood inside the door is rotten, I just had some success with products made by The Rot Doctor here in Seattle. I was able to introduce their penetrating sealer through the existing screw holes, and then follow up with fiberglass resin pumped in with a caulking gun. It worked well enough that I was able to eliminate the oversized screws that someone had used previously and revert to the original #10 screws, and they all seem to be holding fine.
If you need more detail, just ask.
I've also found that most of the little fittings (water tank drain valve, electrical
pass-through, faucet) are standard RV stuff, easily available on the Web. I've been dealing with Dyers in Texas, mostly because they were sly enough to include a printed catalog with my first order.