….that enormous one piece nose/undercab/bulkhead "cap" doesn't look to me like FRP / chopped strand (looking at the product obliquely, in your photos)….it could be a thermoset molded piece, lighter than polyester chop strand FRP.....but I could be wrong.
….when (and if) you follow the manufacturer's suggestion, and cut a hole from under the mattress area, rent a micro-inspection camera (like a sea-snake or similar), and have a look at just what is under/inside that sagging "cap" gap, and try and ascertain if moisture may be getting in there.
If the back of that molded cap has any delaminated organic material bonded to the backside, any additional bonding you spray in that inspection port may just delaminate again without additional/further measures. You may need to fasten that C-channel along the undercab framing after you apply any adhesive, as insurance against future failure.
