After moving the fresh water tank, we had some holes in the floor to deal with. Some were small screw holes, one was a 5/8 inch hole where the drain line had been.
I've used the same marine epoxy with non-blushing hardener for a number of years. I mixed up a small batch and made sure each of the screw holes was soaked with resin. I also coated the walls of the larger hole and sealed the bottom of it with masking tape.
I added wood flour (from the filter on the sander) to the remaining resin to thicken it to a frosting consistency and filled the hole with it, making sure to work any air out of the resin in the hole. After it's cured for 24 hours the masking tape on the bottom is removed and we have a waterproof repair to the old drain hose location.