So, here is my two cents. I have repaired, rebuild and generally fiddled with a bunch of truck campers, both with panel and moulded fiberglass exteriors which gives me only enough experience to be generally dangerous.
I think the first thing to do is probably decide on how extensive of a repair you want to make, considering that when you open the box, it is very difficult to go back or stop halfway. Without knowing your intentions, the questions I would ask of myself at this point are; how long do I want to keep this camper? , is there a risk of a catastrophic external failure if I don't fix this? , if I can arrest the moisture intrusion at this point, will the camper continue to be sound into the future (assuming it is now)? etc. Mass produced RVs are some of the most poorly constructed and ill thought out large consumer products you will find, with truck campers being right up there with the leaders. As everyone knows, wood is an amazing construction material but you cannot seal it into interior corners with no ventilation and then allow it to be continually wet due to poorly designed and executed trim. It will always rot in these cases.
In your photos, you clearly have other issues with the failed seals on the vertical and horizontal trim corner pieces which cover the edges of the panels. It looks like the area with the pulled screws is not overly structural or weight bearing per se, the screws as far as I can tell, only keep the bump in place. The failed seals are probably the source of the moisture that has caused the rotting. Those aluminium extrusions with the channel for the plastic trim to cover the screws which hold edges together, are right up there with the dumbest design winners. They make a perfect direct internal conduit for the water which inevitably gets in under the plastic trim and through improperly set/stripped screws. Of course, if you try and caulk the heads of the #8 screws that hold down the piece of trim, then the plastic doesn't fit properly and becomes fiddly, death to the profit margins of the quick turn around assembly line.
So, Plan A could be simply to pull off the failed trim and re-caulk it properly, using a really good product like Sikaflex. Then you could clean the mating surfaces of the fiberglass and the plastic bumper, rough them up with some sandpaper and then re-attach the bump stops back in place using a VHB two sided 3M tape. This is super stuff and would easily keep the bumper in place and especially prevent any lateral movement. Then caulk around this piece and fill the screw holes with caulk as well. Or, you could gingerly drill out the failed screw holes/underlying wood with an oversized bit or hole saw, say at least 3/4" and then stuff the resulting hole with as much epoxy soaked fiberglass matt as it will take. You can then screw the braces back into the new substrate but more importantly, you can determine more accurately where the underlying wood is on the "mush" scale which may help you with your further plan.
Plan A would not in anyway address the underlying problems which if you chose to tackle, will be a pretty substantial project, the scope of which will only become apparent once you open the area up.
Plan B would be more extensive obviously. You could consider cutting out the bottom foot or so of the entire front piece to get at the failed wood, to start. You maybe able to buy pre-made composite panels or you could take a run at making one yourself. There is lots of good information here on EP for home brew panels. But in closing, remember and I speak from experience, once you open the box, you open the box......
