Depends on how much floor is rotted and how much work/money you want to sink in it. I had an area about 2 square ft that I used a 1" hole saw to cut a bunch of holes through the top
fiberglass layer, then dug out all the bad wood and used fan with a heater to ventilate and dry the area for about a month. Be sure not to drill through the bottom layer of
fiberglass. By jacking the front of the trailer up or down I was able to flood the remaining perimeter with wood preservative and after letting that dry used nearly a gallon of smiths penetrating epoxy, a very watery epoxy that soaks in very well. After that I filled the remainder with regular epoxy with milled
fiberglass and filler to fill the remaining voids. Very solid now. You can just make out the holes I drillled near the front of the trailer