I would suggest a 14 X 14 Door, raise the ceiling to 16ft; you will be able to work on the roof of 13’6” RV’s and your garage door operators will fit more easily. Suggest the building width 20ft, length 50 for resale
Slope the floor to a drain for washing your RV.
Use 2” foil faced insulation on the walls and ceiling to make a cheap to quickly heat building. Our winters regularly get into the low tens and sometimes single digits. I set the heat at 40 and it rarely runs. I can bring the inside temp to 60 in about 20 minutes with the propane heater. Cost to heat last year was about $265.00.
Rough in plumbing in a corner for a hand washing sink and maybe a toilet. It saves me from tracking grease and dirt into the house.
Think about wiring for a 220 volt air compressor.