There is a product called PC-11 (a 2 part epoxy) that is really good stuff. It will even cure underwater and is unbelievably stronger than most other epoxies. I used it to bond a riser (wood to fiberglass) for our roof vent to the roof of the Boler
, and also to bond a frame to the inside of the door (metal to fiberglass).
Also, we change our layout to add a bath with shower, our posts on this might give you some ideas. And the PO added a Dometic AC to our Boler but instead of reinforcing the roof he just added a layer of plywood between the roof and ac to distribute the weight
but the wood was rotten when we got it causing a slight sag in the roof allowing water to pool up so we removed the unit and put a window ac under the bunk. I believe the roof unit can be done but the roof does need to be reinforced properly to hold the weight to prevent the roof from sagging, so it really depends on what lengths you are willing to go to in order to add ac. Window unit is probably the easiest route and the least expensive but does use up some storage space which is a valuable commodity in a small fgrv.