It appears that the root cause of this is there's been no preventative maintenance done. It's important to check all the caulk lines on your rig (including the roof) a couple times a year especially if you're traveling in it.
You may want to first prevent any further damage by re-sealing any cracked or broken caulk lines with something like Geocell Proflex.
It comes in White or Clear 10 oz. tubes and if you need a lot of it, also comes in a gallon can which is brushable. But you need to clean & prep the surface if you want this to last.
On an RV that's that is this old, rebuilding this would be very costly and probably not your best option.