There are several possibilities here. First, Scamp
recognizes the problem , saying that the fiberglass "relaxes" over the years, especially in hot climates. They recommend cutting some oak strips to fit the correct contour, and screwing through the door to mount them. You might create a towel rack or shoe rack or something and incorporate it into the stiffeners.
Second option - Scamp
sells replacement doors. I think they're a couple hundred bucks. So, balance that against how much time you are willing to put in.
But, before you do anything, get out a step ladder and look at your roof. Does it sag? Older eggs especially are notorious for sagging roofs. If it is sagging, that is probably the problem, not the door. My door looked just like yours, maybe worse when I got it. When I replaced the front bulkhead (I have a front bath), and pulled the fiberglass wall back in to mount it all up, lo and behold, the door fits! What happens is: Think of a balloon. Put it on a table. Now, put your hand on top and flatten it. What happens? It gets shorter and wider. The same thing happens to your Scamp. As the roof sags, the camper bows out at the belly-band, and the bottom of the door pops out. It only happens at the bottom because the egg structure ends at the bottom of the door, and has a flex point there where it meets the floor.
How to fix it? The best fix is to remove the sag from the roof and pull the walls back in. That may or may not be practical. Re-contouring the door is another option, but it is really a band-aid, and doesn't address the cause of the problem. I'm assuming you have a front sofa? If so, consider cutting a profile board from oak or similar hardwood that will run from floor to ceiling, shaped to pull the belly-band in a bit. Mount it on the hinge side of the door by screwing through the metal door-mount post embedded in the wall (The metal post is supposed to prevent this problem, but isn't quite stiff enough).