I will also attest to the need to build for the future. My dad built for their old Class C. When they upgraded to an A they had to raise the door and lengthen the building. When they upgraded to a pusher, they were limited to the size of the garage.
My garage is 26' x 52' with 16' sidewalls. If I were to do this again I would go a little longer (truck and 5er just fit in when they are attached since I now have a crew cab long bed. I also would go a little wider as I put my boat and 5er next teach otherer, and it is very tight to get them side by side when the slides are out.
Also, install lots of windows. Natural light is key.
Here are some photos of mine.
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