We had a
Trillium 4500 some years back and as we lived and camped in South Florida, decided to put a Coleman Mini-Mach on the roof. That thing weighed +/- 70 lbs and while it took a year or so, the roof eventually did sag from the weight.
So when we upgraded to the 5500 we have now, the AC was installed by a guy with a clue. The attached shows the steel ladder (painted black in the photo) he fabricated to support the unit. The air conditioner places virtually none of it's bulk on the
fiberglass flat of the roof but is instead supported by the structure which distributes it to the sides. We've had zero sag and zero problems in the years that we've had this set-up.
I'm not sure a Scamp or Casita can be thus rigged as they don't have the distinctive
Trillium "Hump" but it's at least food for thought. Some similar device may work for you.
Doug