The Forest River Work and Play video on the web states that they use marine grade plywood on the Work and Play toy haulers. I got to know marine plywood and fiberglass application pretty well, when I purchased a 34 foot wood cabin cruiser in 1998. I had the boat for about 8 years and almost completely rebuilt it. I did a lot of reading on marine plywood and fiberglass and had some free help from the guy at the local marina. Marine plywood is approximately 3 time more expensive (depends on where you buy it) then regular grade plywood because it does not have any voids between the plywood layers and it uses a water resistant polyester resin to bond the layers.
There are a couple of ways I would make a repair if my 30WR walls were delaminating.
First, I would check to see if there was water infiltration that caused the delamination. If water damage was found, repair the area where the water was coming in. If water was not the cause of the delamination then it could be a bad panel, possible manufacture defect, who knows?...lemons occure.
I would find the top of the bulge (void) and make a mark an inch down from the top to drill a hole. Buy an empty caulk gun cartridge or other means of injecting glue (epoxy or resin) into the void area. Drill as small of a hole as possible, but big enough to inject glue. Now find the area on the outside of the trailer wall approximately where the bulge is inside the trailer. You will need to brace the outside of the trailer wall because you will need to put pressure on the inside of the wall to press the void together. I would back the trailer beside my house (brick cased house) as close as possible and use the house as a bracing point for the outside of the wall. For bracing the outside wall I would use a piece of plywood, a little bigger than the bulged area (the thicker the better), to place on the outside of the wall of the trailer. I would use some type of material to use between the plywood and trailer wall to keep from damaging the wall finish. Cut a piece of 2x4 to be used to brace between the house wall and the outside wall of the trailer. Cut it about 1/2" to 1" bigger so that is can be used to wedge the 2x4 between the house and trailer walls.
On the inside I can only think of two ways that I would brace the wall to compress bulge. First way is to get a cut a piece of plywood a little bigger than the bulge to use on the inside of the wall as you did with the outside. Match up this plywood up to the bulged area where you drilled a hole to inject the glue. Drill a hole in the plywood to match up with the hole in the wall. This hole needs to be there so when you apply pressure to the bulge the excess glue hs a place where it can be expelled. Remember to use something to protect the wall finish and the floor area from the glue. Now measure the distance between the inside walls. Add approximately 1 inch to that measurement. Use this measurement to cut a piece 2x4 to use to brace the inside wall against the opposite wall.
Now brace the outside wall, but did not put too much pressure on the wall but just enough to hold the wall from moving outwards. Use whatever you have to inject the glue and inject it into the hole in the top of the bulge. Now put up the plywood over the bulge and line up the holes. Use the 2x4 to brace the plywood against the wall being careful not to use too much pressure. As pressure is applied, excess glue will be expelled out of the hole. Wipe away ALL excess glue. Let it dry for 24 to 48 hours, depending on the temperature. Remove the braces carefully in case there is dried glue between the wall and the plywood.
The second way. Use the plywood you cut for the inside of the wall. Next, get a piece of angle iron, 2x4 or anything straight and ridged that you can mount across the bulge. Cut this wider than the bulge and drill one or two holes at each end for screws. Mount it to the plywood you cut. Use screws to screw the brace in place. Make sure you measure the screws so as they do not penetrate the outside fiberglass skin. I would use a larger diameter screw with course threads too great a bigger bite into the wall. Slowly screw the brace in place and wipe off all expelled glue. Let dry for 24 to 48 hours. Remove the brace and fill the hole you drilled and the screw holes. This method you do not need the bracing on the outside of the wall.
As for the glue, I would use West System (epoxy), another type two part epoxy, or polyester resin, the same that is use in laying fiberglass. I would not use a wood glue as it does not have the bonding strength.
If you do not mind the screw heads, you can just drill the hole, full with glue, and use numerous screws to screw the void together. I do not recommend using nails as they won't have as much holding power.
By not filling the void in the wall panel it will just keep getting worst due to the warming and the cooling of the air inside the void (I learned this the hard way). In the summer the air will get hot inside the void (if it has nowhere to go) and expand, delaminating more of the panel. As the weather cools the pressure will go down. This will repeat as the weather changes and the void will get bigger.
Just so we are clear, this is just the way I would do it. The above is just a suggestion. I am sure there are many ways to do it...the best way is to replace the wall, BUT $$$$$.