Bathroom is for those times when you don't want to use the coach facilities. I don't really need it for my setup since I have both in a nearby shop. If you are going to run power, go ahead and put in 50amp service. My facility is 60' long - just long enough to have my toad hooked up to my 39' coach. I seldom do that, but there have been occasions when it was very helpful. If you have a concrete floor, make it about 14 feet wide. I had a 10' X 60' slab poured, and could really use the extra two feet on either side. My basement storage bins extend with the slides.
Where in Oklahoma? You will definitely need to winterize. I'm in SW Oklahoma, and it gets below freezing several times in the winter.Seldom stays that way, but it does happen.
GM