Seven years I had a building built 14" tall so I could get a larger RV at some point. I knew nothing about doors, floors and sidewalls, but I told the large building company what I wanted. Bottom line...
You need a builder, not Cleary! It is likely that you can get that fifth wheel in that building cheaper than the 10 foot extension. Call a builder before you move forward. Take my advice, you got nothing to lose, if the 10 feet is cheaper than you go with it. I believe there may be a cheaper option depending on your roof trusses.
If it is truly a 14 foot building, Yes You Can get a higher door, you just need a different header. Will cost but it is cheaper than adding 10 feet.