A 12 x 14door is the standard Drive in Bay door for industry. If I were to build an RV Garage, I would build it minimum 50 foot long. The RV bay would start about 8 feet from one wall out a minimum of 24feet, then I'd extend out past that another 26 feet minimum and install three 10 x 9 doors in three bays leaving about 14 feet x 26 feet for a work shop. OK, 50 x 50 with a 20 foot ceiling in the RV bay with a cat walk and a trolley to attache fall protection to while working on the roof. In the other half of the building there would be a second floor. More work space or an apartment.