On my 1999 17' Cassita there's a way too large unsupported area for OSB just in front of the rear table.
From underneath you could see the fiberglass bottom had sagged, the OSB on a Casita
is not glued to the outer fiberglass. I selected a piece of angle iron the same size as adjoining pieces, jacked it up even with a bottle jack and welded one joint at a time with my MIG laying on a creeper under the jacked up trailer. I had the garden hose handy and sprayed each weld off quickly as I went. Did the same to the other side and horizontal joints. Cooling each as I went. I reduced the spanned area by one half.
The only sign of me welding it in was a little stain from smoke that easily cleaned off.
I obviously did this on a warm day.
Then I drilled four 3/8" holes just through the outer fiberglass in the area and sprayed in foam and covered with duct tape while it cured and that bonded to the deteriorated OSB and it turned out super.