Welcome to the U-Haul
Those of us in the upper Midwest are jealous of those who can keep tinkering on their camper projects straight through the winter... mine's all wrapped up for the winter. [with a lot of fixing-up still undone
Floors getting soft is pretty common. If it's just in one or more distinct areas, you should be able to splice them in--sealing the patches with f/g resin above and below. Replacing the whole floor has been done by several folks, too, and you should be able to find procedures, step by step, for taking the body shell [after bracing it] off the frame... and then digging out the old... installing the new floor in 3 or 4 sections.
These old windows
are quite leak-prone... even with the good, step by step rebuild instructions it's quite a chore to get them completely leak-proof.
Fiberglass repair isn't as hard as it might seem. Messy, for sure, but a long way from rocket science. "Fiberglass Dave" has a long write-up on here on tackling it.
There's quite a bit of good material stored in the "Document Center"--accessed at the top of the page. There's also lots of good info & helpful folks over on the U-Haul
Facebook site... if you're not opposed to Facebook. A number of U-Haul
owners post both here and over there.
At any rate--ask any questions you have as you go along, and there should be plenty of folks chiming in.
Gary [in Iowa