Are you really handy? And have a few good friends? In my slightly younger days I bought a prefab kit from Mueller Steel Buildings. 30'x40' (1'200 sq ft), 4-12 pitch roof, 2' overhanging eaves all around, guttered, fully insulated, three windows, two roll up doors, and one walk thru door, on concrete slab foundation (with extra steel) for about $15,000. This was for a shop and farm equipment. But higher walls and taller doors and it would have been great for an RV. All bolt together, no welding. Lot of work, but a satisfying accomplishment. Prices would be a little higher now. But the extras also added a couple thousand to my price.