The membrane was poorly installed on the rear corner of our unit. It was trimmed to short to cover. We had to replace 16" of plywood. Original construction is flake board over wood trusses. Was told by dealer that this was an aluminum frame. Was a lie. Just the latest in a long list of non warranty expensive repairs. See my profile for a list.
Good ole Dutchman blind installers. Looks like you have the right idea with the repair.
A suggestion, if you have it that open it wouldn't hurt to beef up the top ladder supports while you're in there. But it might be scary taking a look
2013 Voltage 3800, 2012 Chevy 3500 HD
2010 Yamaha V Star 950
2009 Yamaha Raider
Volunteering with Habitat for Humanity Caravanners
Thanks for the idea of extra structure for the ladder. It was 93 degrees when we repaired this and just didn't think about it. I did use heavier plywood instead of flake board. And the bottom ladder mounts are on the 14" by 8' diamondplate which holds the rear wall. This was added a year ago because you could put your hand through the crack behind the rear couch.