Tires seem to be almost a religious issue. I drive mine once a month to avoid flat spots and place a piece of plywood under the tires to help abate moisture. Some also use rubber floor mats. Basically any kind of moisture barrier until the concrete is poured. I yield to these guys but do not believe you have to spend a lot of dollars to be effective.
Mine sits under an RV carport also...dirt floor. I poured a couple inches of gravel in the areas where the tires contact the ground. I was going to pour sealed concrete but now we are selling the house. Dirt will dry rot your tires quickly...so will unsealed concrete.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie