I don't have a Scamp (the Boler
has a slightly different, fiberglass floor), but as I understand it that section of floor is tabbed (with fiberglass) to the shell.
Rather than raise the floor 3/4" and deal with the extra weight
, height problems, etc., I would cut the rotten floor out and fiberglass tab the new one in at the same height. You can use marine plywood, or perhaps an exterior ply - and you could seal either with epoxy resin if you wanted to. That said, keeping freshwater leaks
out of the interior is the main thing.
I know a number of people here have replaced that back section of Scamp flooring at the stock height, using fiberglass (cloth in "tape" form plus epoxy resin), and it should not be too difficult, IMO (probably easier in the long run - and better - than dealing with all the side effects of raising the floor).