If you are referring to my photos, those are mounting screw holes. I just laid the frame in the opening to take the "before" photo while I added the lip..
Mine also had a membrane and it probably kept water from getting into the walls for years. But once that water got in the frig compartment, it stayed there because bottom of the frame is higher than the floor of the frig compartment. After years of soaking, water finiall found its way betwee the walls and ... the rest is history.
Someone who buys a new MH and keeps it for 5-7 years probably won't see the negatives, but I guarantee, when that wall starts rotting, it will severely impact the re-sale value.
I've checked mine since I did this mod. We had 5 inches of rain a couple weeks ago in 12 hours, heavy, heavy downpour lasted all day. I popped the frig cover, dry as a bone. :-)
2001 Coachmen Mirada (Ford F53 6.8L V10) - Toad 2001 Civic
It won't do MACH 2, but I can get a sandwich and take a pee.