I'm guessing that it's that lowest edge trim panel which for all intent, hides the frame.
If it is that very thin piece of material, it will need to be supported inside once you make you repairs. While it probably isn't fiberglass but another plastic material, it can be repaired using fiberglass cloth if prepped correctly. I would, if at all possible, remove the cracked piece from the trailer - yeah, I know, a thousand screws holding that trim piece on with a lengh of aluminum with vinyl filler - but you need to clean and prep the backside with 220 to 320 grit sandpaper for a 'tooth' to hold the resin soaked 'glass cloth or matting. Once you get the piece repaired, the exterior will probably need to be painted. If you have the tools, regular auto body prep for paint will work. If you don't, rattle cans are always a good fall back with my preference being Krylon as Rustoleum is, IMO, way overrated and takes way too long to dry. OK - you've done your repair so now it's time to put that piece back on. Those many screws, a roll of butyl strip seal and the panel is back on - but you need to support it from further vibration. Some bent aluminum angle will work best - but you need it to stop flopping around in the wind so whatever works, screwed in place.
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')