Some good advice previously posted. 50 amp is a given.
I would also say build it big enough for your next two rv's length. You will always go bigger until you downsize. lol.
I would also add ventilation fans so that you could exercise your generator and engine without relocating out into your frigid winter.
I have a 40 X 60 x 14' for the camper. Wish it were bigger, the garage too.
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