Do the 14' door.
Your RV now is a travel trailer, which is why you only need 12 feet.
If you ever decide to go to a fifth wheel, it is very easy to go over 12' in just about any fifth wheel that offers a full height bedroom over the hitch, which is most, if not all, fifth wheels these days. I don't think they even make rv fifth wheels anymore with less than 6' front bedroom cieling height (except for living quarters horse trailers, but those are mostly goosenecks)....
As an alternative, if cost of building the garage is an issue, you could buy a pre-made 12'w x 16'h x 30-40'l metal awning building. They are far less expensive than building an actual garage. The roof sheet metal us usually wrapped around the corners and goes part way down the sides. It can go all the way to the floor if you want it to, for not much more money. The rear end can be enclosed too, for not much money. Then build a pair of swinging doors for the front and you have your RV covered and safe, for a lot less than constructing an actual garage. The awnings can sit on dirt, or asphault, or a concrete slab, your choice. Obviously, concrete is the most expensive choice.