of the door handle banging against the inner latch mechanism as you bounce down the highway is the likely cause of handle sag. I have always understood the outside handle to be the primary culprit, but the bungee cord post implicates the inside handle...
One solution is to mount the outside handle so it points down. That way bouncing does not produce a rotational force on the handle.
One person recently suggested an alternative: mounting the handle facing backward so it balances the inside handle.
Having the door pop open could have a number of causes. I think the most likely is the tongue on the latch not fully extending behind the striker plate. I push hard on the door to fully compress the seal, and when I rotate the handle to the closed position I try to feel whether the latch encounters any resistance. A couple of push-and-tugs, lock the door, and go. I've never had the door pop open, and I've been on some rough roads.
Regarding replacement parts... If just the lock fails, you may be able to source the outside handle locally and avoid Scamp shipping charges. When the lock failed 24 hours before our maiden voyage in our new-used Scamp, I found an identical lever handle at my local RV shop. In a pinch, a square-shaft garage door handle (T-handle or lever style) will work. You'll have to cut the shaft off flush with the inner latch as they're made for thicker doors than the Scamp.
If the inside latch is bad (less common) you'll have to go to Scamp or a vintage trailer supply site. With Scamp it's always best to anticipate future maintenance and order a bunch of stuff at once so the shipping doesn't eat your lunch.
Which reminds me... I need to get a Scamp order in myself...