rear wall repair sprinter 5th wheel

[akaiser]

hi:

2004 27 ft keystone sprinter 5th wheel.

We have had a leak in the rear passenger side corner for some time now and i've decided to try and do the repairs myself. We've started the tear down and the water damage is not too bad. The rear wall, however, looks to be toast. My plan is to remove the fiberglass exterior panel and rebuild the wall. I have several questions and appreciate any help.

1. The outside portion looks to be fiberglass with a cardboard backing. The backing now looks like paper. I want to replace the cardboard portion with luan and use constuction glue to attach it to the fiberglass. Will this cause any issues with flexing - temperature and driving?

2. We've had this camper for several years (over 30k miles!) and as far as i remember the back wall has always been separated from the floor. I want to attach the bottom of the wall to the flooring joists when we re-frame the wall. Were these built unattached on purpose for flexing or has mine just come loose from deterioration?

3. I want to use a rigid insulation in between the studs and glue the insulation to the luan. I want to make the back wall as solid as possible. Again, are there any issues with the trailer flexing while driving?

I appreciate any feedback.
thanks