I'm a little surprised your Tuscany does not have an insulated subfloor the tanks set on, but use at least 5/8" marine ply, with a 16 gauge galvanized sheet metal cover on the bottom for protection. You can either use contact cement, or construction adhesive to attach it to the ply. If you do not have a fresh water tank drain now, I certainly install one at this time. Don't worry about the size of the tanks, but how much water they will hold. Use 8 pounds per gallon for weight that the floor must carry.
