Sorry for the spelling dyslexia mess up. "Dead axle" Scamp
and Casita's use torsion axles. These axles have rubber rods inserted into the axle
housing to absorb the road shock and act as a spring. After usually 15-20 years the rubber becomes compressed and looses it a ability to absorb the shock of road ruts and potholes. There are several threads on how to check a torsion axle
I usually deal with rotted wood in the door frame ear extension. When I replace the wood I usually make the wood a tight fit into the metal channel and solidly attach the wood with 4 screws. I then re-glass over the wood block, shell, floor bottom and metal frame. The key is to ensure everything concerning the door is aligned before glassing things together. The glass work in this area is never pretty. If the floor near the door is rotted alignment is critical. If your front under seat floor is good and solid your door frame alignment is pretty much set.
Just verifying you do have a piece of angle iron under your door walkway near the outside of the trailer attached to your walkway lower metal frame? This prevents spread of the lower door opening.
It's hard to say without viewing your trailer but if you have upper door corner cracking on a 70's-90s Scamp with a solid frame and door frame support I would really be looking at an axle.
This is how it usually goes a bad axle transfers road shock to the frame and support members, they fail then you start seeing shell cracking at the door upper corners. Or the upper floor rots at the edge of the shell and the shell settles causing cracking. All being said being said upper door frame cracking is pretty rare on Scamps and Casitas. It's usually a individual specific trailer issue. Also gelcoat cracking in this area and others areas can happen and just be a cosmetic issue and just be monitored.