Our Motorhome Garage
First I'll give you the dimensions and then some further modifications if I were doing it again.
Our MH garage dimensions are 50 feet long and 18 feet wide. The inside ceiling is 16 feet high. The door is 12 feet wide and 13 feet high (the door header gives 13 feet overhead clearance). The sewer drain is 18 feet back from the overhead entry door so it is in close proximity to the coach's sewer drain. There is one 50 amp and one 20 amp electric socket each on their own breakers. I also have water so there is water, sewer, and electric. We live is Las Vegas so we installed an air cooler (swamp cooler). There are 12 two-tube florescent lights around the perimeter of the ceiling 30 inches in from the wall. Do NOT put them in the middle of the ceiling. You will get poor lighting if you do because the height of the MH will block the light. The overhead door is insulated with styrofoam insulation. The garage is insulated. There are three windows on each side of the 50 foot length.
If I were doing it with more space on my lot I would make it 60 to 65 feet long in order to have more storage space for other "stuff" at the front of the MH. Also, it would add value at resale if the purchaser had a 45 foot MH. Even though there is plenty of room to open the slide outs and walk around them I would make it 20 to 22 feet wide. Bigger is better and it makes the garage more valuable. If you think that a Prevost owner might ever buy your property, should you sell it, make the overhead door clearance 14 feet because the newest Prevost MHs with a dome type satellite dish require 13 feet 6 inches of clearance. The bottom line is, make it larger when you first build it because it is MUCH more expensive to modify it later.
One more MAJOR point, our MH garage is connected to our automobile garage using a 9' by 9' opening and the auto garage has an entry door directly into the house. After having a MH garage for 10 years we have come to the conclusion that being able to walk from the house into the auto garage and on to the MH garage is the nicest feature of having a MH garage. Any time we load it, unload it, clean it, or work on it whether it's Winter or Summer it's just a delight to have the whole "shebang" indoors.
JerryF