Couple things to also consider:
1. Will it have a bike rack on the back that isn't included in mgr length specs.
2. With the back part being lower than the front, can you still get the front low enough to get it level so you can leave the frig running while parked.
3. Will you have a straight shot to pull out before turning? If not, the back end will swing and need to factor that into the width so you don't hit the sides.
4. Allow for any AC units and ensure they are included in the height.
5. Allow for any truck ride height adjustments, especially on uneven surface when approaching the garage, and also will the door sag at all when fully raised or will you have 14' of true clearance with door open?
6. It's nice having a clear span over the 5er to work on the roof without dealing with rafters.
Just remember, once you get that 5er home and it doesn't fit, not a good situation to be in. I would go bigger than you think and make it easy to get in/out. I have a published 39' 5er with bike rack extending to 42'. Put up 45' garage, which is fine, but wish it was bigger for more space walking around.