I'll chime in on some of this. I've not the space to build where I currently live, but plan on building when I move. Regarding the Garage Door and the rest of the building design I've spent 45 years in the garage door industry, and concurrently in the building industry. I've seen a boatload of garages and work shops and more than a few failures due to poor planning. So here goes.
Door: 12-0 wide MINIMUM x 14-0 High. This is the FINISHED OPENING that you specify. The door can actually be ordered 2 inches wider and an inch taller to overlap for a better seal. Stop molding is generally 1/2" thick so don't worry about that. Your ceiling MINIMUM HEIGHT at teh sides of the opening and all the way across is a MINIMUM of 18 inches higher. Make your ceiling no less than 15-6 this will give you just barely enough room for a garage door opener. make it 16-0 and you will have no issues.
Building width: BIG and BIGGER! Seriously open up the slides on a modern RV. The base vehicle is 8-6 wide, add in two nearly 4-0 slides and 3 feet minimum walk around, your RV bay needs to be 22-6 wide! As for length, a minimum length of 50 feet will just barely allow you to walk in front and back of the largest RV. It is cheaper to build it bigger then to make it bigger later! I'd go for 55 to 60 feet in length.
As for amenities in the bay: A pit if you do or plan to do your own service. DO as a previous posted said and have an angle lip built in so you can put in a 3 x 12 removable deck. 50 amp RV plug on a separate circuit. Water, sewer and an exhaust system for both the main and the genny. A concrete floor, minimum 4000psi, 6 inches thick with heavy mesh reinforcing. Heat, you want to be able to keep the building above freezing. Rally good insulation helps a lot!
A door at both ends is really nice if you have the space, but remember you also then need that much more hard driveway to use it. that adds considerable to cost.
If you don't have a garage for your cars, you can add a 25 foot deep 2-story bay down ones side, put in three 10 x 9 garage doors, make this ceiling 11 feet high you can now park any normal car or truck as well as your tractor inside. This of course only leaves you 5 feet above if you are looking to keep the ceiling flat across the building, however your architect can make this side taller allowing you to put in an apartment above the car garage. Watch your zoning here. not every place will allow this and you may also run into a number of additional concerns for fire protection having living quarters above and next to a garage. If you decide against the apartment, you can easily lower this half of the builging and save considerable money.
This concept has you building a fairly large barn, about 3,000 sq. ft. 50 feet wide and 60 feet deep. Next to the automobile parking where you have the three doors you'll have room for a 'workshop' that is about 20 x 25. The space between the three garage doors should be 2 feet. and an additional 2 feet at each end. or 38 feet of garage. I'd then install two additional garage doors, perhaps three. One from the garage to the work shop, one from the work shop into the RV bay and possibly one from outside into the workshop.
Best of luck to you.
