One thing to add on the FRP... I would not use the stuff from Lowes because I don't think it's really rated for outdoor use. I could see that stuff yellowing and getting brittle over time if left to the elements. It's usually the product you see lining a bathroom wall or some other similar interior space for easy cleaning.
I would suggest contacting your local RV dealer for FRP because that's the stuff you'll see used on RV's. It usually comes in an 8.5' roll in white with a slight fibered texture. This is what I used when I rebuilt the roof of the truck camper we used to have (start on post 47 in my thread). If I recall it was around $10 per linear foot at that time (at 8.5' wide) and since it was local there was no crazy shipping fees involved. That was ~ 2014 though.
If your issue was just on the corners I would have said use some eternabond tape to seal and bond a piece of aluminum trim on. Since yours is on the sides, I'm in agreement with some others here in saying you should probably strip it off and bond on a new skin. You could use aluminum sheeting just as easily as FRP. Bond the sheeting over your plywood sides, use a router to trim the edges and add sealant and trim to the edges. If you wanted to paint or use a spray liner after that is up to you.
As for getting the old stuff off, I would first contact Raptor liner for some advise. Of course you could always try the heat gun and putty knife trick. If that doesn't work I would look into using an angle grinder and some sanding pads to mechanically strip it off. Good luck.