My 2010 213EXP had a soft spot in the floor where there was a seam in the flooring. Of course, this was right at the entrance door to the trailer so it was a very high foot traffic area.
Since my 2010 is now out of warranty, and the soft spot getting worse, I took action.
I bought cut a piece of 5/8" plywood to width (about 21" wide) by a length of 48".
I also bought PL construction adhesive (for a caulking gun) and covered one entire side with this adhesive. I then put the board up in place and held it there with a automotive floor jack.
I then cut 2 pieces of 2x4 lumber 21" long each and covered one side of each with PL construction adhesive. I spaced them 24" apart.
Then I cut some metal angle to support the 2x4s and screwed them in to the cross members.
Once complete I sprayed everything with automotive under coating (in a spray paint can).
I allowed the adhesive to cure 48 hours. The floor is now rock solid. We went camping for 10 days straight about a week after fixing the floor and the floor has remained solid in that high traffic area.
I've attached a picture of what the underside of the trailer looks like now.