The two biggest mistakes I've seen on RV Garages is door height and length.
I cannot believe that builders think a 12' tall door is enough for a RV garage. They are just idiots if they think that. 14' is the minimum height as we all know.
I shake my head when a builders makes the length 40' because a 40' coach won't fit. Make the inside a multiple of 16" and always be over 40' or 45'. That would make it 41' 4" or 46' 8" the minimum I'd EVER make the length.
Width should never be less than 14' with a 12' door and then only if you never ever want to open your slides. I'd make the inside at least 20' and more if you have the room.
If you're building with the gable end where the door is, always leave extra room for both the front and back. A gable end building is fairly easy to add length to.
My dream garage would be attached to the house. The house would be two stories both floors having at least 9' ceilings and a full foot thick second floor.
With the RV garage attached and it's floor a foot below the main level that would give it a 20' ceiling height so you could stand on the roof of a 13'6 6" coach. The width would be at least 24' and the length at least 55. Even if it wasn't a drive thought I'd have a standard 8' roll up door in the back.
I'd also run a 3' wide catwalk around at least one side of the garage so you could walk out of the second floor and onto the catwalk then drop a bridge onto the roof of the coach. That way you have easy access to the coach roof for maintenance.
I might even waterproof waterproof all the walls and have a floor drain so you could wash the coach indoors.
Lastly I would have a winch rail attached to the ceiling so you could not only run a winch from front to back but attach a safety line to it as well. Now that's an RV garage