Yep, mine looked just about like that too. Funny thing, there was bondo in the front edge of my driver door filling a dent that must have been the same size as the dent in your door. Both my fenders had that rust hole in it there above the fender extension too, that is where the plate that holds the fender to the cab sits, its about 4" wide and sits horizontal so it can hold anything that gets up there.

You know, if you really wanted to use that truck to pull a 5th wheel trailer, can you replace your axles with ones from a newer truck and keep your old frame? If your springs are OK use them, or maybe you can modify the spring brackets that match your axle. Then install the drivetrain of choice, upgrade as needed. Gene