Keith, my Trill 1300 has had a stainless steel door holder on it for four years. The receiever is on the lowest part of the door on the "ridge/ accent line"...that being the thickest part of the shell where the top and bottom halves overlap. I used stainless pan head self -tapping screws and slathered on epoxy on the
fiberglass. The tongue part of the closer is also on that double shell part and is likewise attached. To stop the tongue from flapping while on the road, I screwed a small rare earth magnet in a steel cup as a retainer. (Lee Valley tools...on the web..."Rare earth magnets")
To date all is secure-even in the gustiest of weather, and after two West Coast, two East Coast, and three Florida trips.
Pre-drill the holes slightly smaller than the self-tappers, you could use double face tape to pre-position the rig, breathe deep...measure twice -drill once...and presto! No more door blowing off its hinges and tearing up the trailer wall.
Alistair