Words of wisdom: Confucius say, many men smoke but fu-man-chew.
Here are some suggestions to your floor problem:
Cabela’s Marine Catalog, 2006, page 79, has some marine grade “Git-Rot” 2 part liquid epoxy that says it will penetrate rotted wood and restore it to original strength. Item # ST- 01-7641 @ $30.99 USD
Note: To keep it from leaking through to the ground, you will need to put something on the underside to retain it.
If you could epoxy a
fiberglass mat to the underside, let it cure then use the Git-Rot on the topside then you might have a stronger floor. You could do this by cutting a peace of plywood to match the underside that needs to be strengthened, putting wax paper on the plywood then a mat of epoxy coated
fiberglass on top the wax paper, put resin on the underside of the floor then screw the plywood on the underside to the floor like a sandwich (Floor, Fiberglass, wax Paper and Plywood), This way, you would be able to remove the underside plywood after it is cured. Epoxy will not stick to wax paper.
Or
You could just epoxy / screw the plywood to the underside without using the wax paper and leave it.
Then use the git – rot on top and as an extra measure, put an epoxy a fiberglass mat on top the whole mess.
Here’s wishing you luck with the repair.