We also have a 1979 International Sovereign
, center bath. We had similar leaks as you do and floor rot in pretty much identical spots (see my blog for specifics or PM me). We found that there are 4 "corners" that were leaking in ours (look on the outside where the curved ends meet the straight aluminum and those joints that run from roadside to curbside over the top of the trailer). Those 4 points on the floor were water damaged, some more than others. Only one had rotted through and we cleaned the edges up, tried them out and put epoxy over the surface (it was about 4" X4" hole).
The rear floor all the way across had rotted by along the back wall. We replaced a 2' section all the way across with new plywood after fixing the leak which turned out to be the trunk area. Water collected on top of the trunk and wicked into the flooring under the rubrail outside. Water also ran into the trunk and along the bottom of the top of the trunk (if that makes sense) and into the floor. After all of those holes were Vulkem'd, the leak stopped.
None of our leaks came from windows oddly but may not be the case for you. Good luck with your restoration and if you have any 1979 specific questions, feel free to PM me. Although I am new to this, I have perused these forums way too much and have gotten pretty far on our 1979 renovations (hope to be camping this summer).
When I look at your photos, especially the fish-eye pictures, it looks like I am looking at ours after gutting and before we started our renovations.