Adventurebound,
Like others who have posted here and elsewhere I had a PU box eyebolt pull up. This was on the previous truck, it didn't get worse so I just left it. Yours, obviously you can't. I think your doing the right thing with first trying backer plates. I hope for your sake it will be enough with an already damaged bed. And I'll will add a few additional thoughts ...
Are you 'wheeling it too hard ? It doesn't sound like you are out of the usual usage range. My first thought tho is, do you you air down the tires for off highway driving (gravel haul rds etc). I do, and it makes a noticeable difference in reducing shock intensity to the vehicle, at least to our vehicle with our set up. If you have overly stiff walled tires, perhaps a softer type might help absorb the bumps more. Similarly if you have an aftermarket higher spring rate suspension, perhaps an alteration might help (remove a leaf for eg).
Bolting thru the floor - Previous to this FWC we had another type of slide in which was thru bolted. That one went into the PU box formed "cross members" underneath the box floor. One would think that a better placement than just thru the PU box floor but I note on ours 2 of those mounts crushed the cross members. So again, damage can happen. Thru bolting on yours (with a boxed framed chassis?) I would look at using the existing PU box mounting hole locations. I think there are 4 and that they would fall within the footprint of your camper - but they may be inconvenient. I'd replace the oe bolts with similar dia and strength, at least 10mm dia. and use a fender washer/ backer plate on top. I've read some FWC are mounted thru the camper floor. I don't be surprised if the factory makes no changes to the floor pack staple construction but one could add brackets or angle to strengthen the joints if desired.
Stiffening the bed - Previous truck I had a sheet of 3/4 plywood bolted to the floor (using the holes from above slide in), can't quantify how much, but it did seem to stiffen the PU box. Mentioned because some of your damage may have occurred thru twisting. You might even be able to cut the ply shape so the floor eyebolts also attach thru the wood (threaded thru tee nuts for eg) for a little extra support.
Turnbuckles - lots of threads on here to search on this subject, how tight etc. I like using a lock nut or tie wire after tightening so when I check as part of my routine and find a loose but locked tie down it tells me the turnbuckle has not loosened, rather the mounts have pulled or the camper shifted. Useful info. Also, a poster mentioned earlier using external mounts and others have posted about using spring tiedowns. If your issue continues I'd say both are worth consideration, even on only one end of the camper.
Bed mat - I use one and like it. I think it helps prevent the camper from sliding around and it can be the "something" that has to give when the box gets twisted, namely a bit of cushioning. Speaking of camper sliding, some folks have attached blocking or angle to the PU floor to restrict possible camper movement. Using the plywood above might give another option for attachment.
