You're in a Serious Pickle. Stepped in Deep Do-Do.
If I understand it right, there's little more than gravity holding your bus together at the moment. Well, that and the exterior skin sheet metal. Here are a few more pictures from mine to help check whether I'm understanding the situation correctly. The first shows how the rivets attach the roof bows aka hat channel aka ribs to the chair rail. Those rivets are the main thing holding the roof-and-wall piece to the floor-and-frame piece. In my Blue Bird there are four solid rivets holding the end of every rib to the chair rail. A few, like the one here, are places where one section of chair rail and another overlap. These had six blind rivets in addition to the four solid rivets. Only three dashed lines are shown because the top-most solid rivet spot got cut out of the picture.
And a view from the inside. Again, this view is possible because I've removed the sheet metal from both sides of the wall, cut out all the rivets, and jacked what's left of the wall and roof upward 16" so the bottom tails of each rib are now visible above the chair rail. I had a sheet metal shop make me some hat channel that fits just right over this existing stuff to extend it.
So, first things first: there's really very little holding your bus together right now. It'll be a Good Idea to not move it at all, and maybe also to make a deal with the local weatherman to not call for any strong winds. Next, you'll have to figure out how to replace all those fasteners. The solid rivets may have been originally installed with a tool like this (in other words, this is the rivet gun, a sample rivet, and the "bucking bar" I bought for installing new rivets for my roof raise):
Solid rivets are probably not looking like a good option to you right now, though, because bucking (smashing) them requires access to both sides. You probably didn't have in mind to strip off the exterior metal like I have done to mine...
IMHO blind ("pop") rivets are probably not a good idea because in general (again, in my opinion) they're not as suited for structural work as solid rivets are. Though one of the rivet vendors I talked to (probably rivetsinstock.com, though hansonrivet.com is where I ultimately bought my gun and rivets) offered something that was a blind rivet but supposed to be as good as a solid rivet. I thought it was expensive: 50 cents each as I recall..
nat_ster on his Four Season Prime build has used short bolts, 1/4"-20 by 1/4 or 1/2 inch length I think, for fastening new sheet metal to the ribs on his bus. If you have access to an impact gun, you might be able to do this. Drill an access hole through the hat channel above the chair rail, fish a piece of wire or string as a leader into the access hole and out the hole where the bolt needs to go. Use it to feed a bolt, thread end first, into the access hole, down the channel, and out through the rivet hole. Carefully get a nut started on the bolt, then zip it tight with an impact wrench. This is a fun trick for bolting things when you don't have access to hold a wrench on one end of the bolt. If you have access to a MIG welder, you could weld a little bit of MIG wire to the end of the bolt and use that as the leader to fish it down the wall. Or maybe something thin tied around the bolt threads, like dental floss, fishing line, or sewing thread.
As to the chair rail ledge that probably was the start of all this.. I guess you could cut it away.. I wouldn't, though. I think it adds rigidity to prevent the walls wobbling inward and outward. In my bus I plan to insulate deeply enough that this ledge will be buried inside the wall.