My own feeling is that all of these rivets are structural, but there are too few of them to actually strengthen the structure to any extent.
The movement in the two structures and the lack of enough bonding with enough rivets let the parts work against each other and loosen the rivets even more.
I looked at all of the rivets in my 1986 16' Scamp and every one of them was loose and working (leaking too).
The through the hull fasteners were pretty much in that condition because the insulation prevents them from being held tightly together when pulled with as weak a fastener as a Pop rivet. At least the SS screws can draw the parts together. Be sure to use washers to spread the load out to keep from breaking the relatively fragile fiberglass.
The one interior parts with no signs of working and leaking on my trailer were the wood (mushboard) rear over head cabinets. They were held in with wood screws into the particle board. No leaks
and the parts were held in quite well.
I think the deluxe models with wood cabinets are made the same.
When I installed my new oak cabinets (modified from a cabinet discount store) I did the same, using countersunk washers and screws and pre-drilling the holes to not split the oak. I also cut away the insulating (ensolite) insulation along the mating line and used structural glue to bond them in place and make certain there would be no leaks
. The screws were also sealed with a fed dollops of Gorilla glue in the countersunk hole and under the washer and head of the screw. No plastic caps to fail!.
As a result the added cabinets, walls and closets actually stiffen and strengthen the trailer. There are also no holes remaining from the old installation those being gound out and filled with fiberglass filler. (AKA Bondo).
Good luck with your fix. You can buy SS hardware at Lowes, boat shops and Aircraft Spruce and specialty (aircraft).