I have to agree with CapnDan and others who advise to build bigger. I originally was going to build a smaller building, but I wanted room to grow, and to stora a boat as well as the RV. I talked with several builders, and found the perfect match for us. He did an outstanding job, and now, 3 years later, I could not be more pleased. We ended up with 36x48, with 16' ceiling; 16x14 pull-thru doors, and 6" reinforced concrete pad.
I took out a permit myself and did the electrical, and subbed out insulation and sheetrock (5/8"), and paving. I really like it, and recommend you consider building larger than you think you need now.