No structural problem or worries as long as your floor is sollid but no a easy fix either. That section is attached BEFORE the exterior skin is put on. After pulling off the belly pan on ours today I think I know why it happens. The belly pan has rivets into the crossmembers to suport it in the center. On ours most of those rivets have pulled through so most of the belly pan was being suported by the outer edges. As the coach travels down the road I bet a low pressure are forms under the coach. The pulls on the belly pan even more and cause it to move. The movement causes the wear at the ribs.
Knowing this the first thing you need to do is see if i'm right and the pally pan has pulled free from the crossmembers underneath. If it has hit the local metal supply and pick up a 1 inch stip of aluminum and make a reenforcment plate to put over the original hole and rivet through it to make a stronger attachement.
The easy repair for the damage at the out riger ends wuld be to buy some new .025 2024 or 6061 aluminum sheet. cut it into 10 inch stips and put it up to the bottom of the outer skin over the damaged pannel and rivet it on. If you use Olympic rivets from www.airstreamdreams.com
they will look very simular to the original rivets once finished. Line them up under the existing Rivets and only the really observent would notice the extra row and thos same olks would also know why.
I just ordered some aluminum from www.airpartsinc.com
and VERY pleased with their service (same with Airstream Dreams). Super friendly folks and they sell the aluminum by the foot. You might ask if it's possible for them to get a square cut and if they could cut it into 10 inch strips for you. The cut on the material I bought was pretty clean but I havent taken a close look at it yet.
Now the down side. 2024 and 6061 is not cheap!. Air parts beat all the local suppliers by a long shot in price. Trailer trash is saying he is picking up aluminum form a marine place in FL for dirt cheap but has not mentioned what it was. It may be something like a 3000 grade not sure how well it would match the existing skin. 2024 t3 AlCad is what the older units are made out of for sure. Not sure about the newer units. The belly is a lower grade. 2024 t0 (not Alcad).