The curved door not meeting the curved body is no big deal ... any body shop that specializes in fiberglass work can "true" up the fit by building the trailer door frame up to meet the door. If you are handy, you could probably even do it. Fiberglass is easy to work with, spreads like frosting, and you reinforce it with mesh ... and when harden, you can sand smooth.
Concerning the windows
... Hehr is the largest manufacturer and supplier of RV windows ... and could probably supply a window to fit.
You might have to cut the window opening larger.
The moldy cushions means the windows have probably been leaking for quite a while. The non-fitting door means that's probably been leaking there, also.
Thing you should be most concerned about is the trailer's floor and substructure. If it's wood ... and if the trailers been leaking, the floor is probably soft. Check the floor around the windows and door with a small screwdriver (really tiny one) or awl ... see if it's easy to poke into the floor. It won't be easy to push in the screwdriver into a sound floor ... easy to push into a rotten floor.