If its soggy, you likely have a leak you need to replace before you fix the floor. Or are you just saying its swollen like it had been previously wet? Is there anything under the MDF? If so that would rule out it soaking up road spray from the bottom.
Exterior rated wafer board or OSB would work, but personally I'd use marine grade plywood. More expensive than normal plywood, but much less susceptible to future water damage.
Sent from my A0001 using
Fiberglass RV mobile app