The easier it is to access the more likely you are to use it
I looked at your diagram and it would work if you were only going to store the rig and weren't planning on using it. Having to find hefty guys to help in addition to purchasing the dollies would seem to tip the scale in favor of another solution.
From your diagram with the top of the page being North, I'd build an addition along that North Wall the width of the the unit plus enough on either side so you can extend the sides and still walk around / work easily. I would probably also build enough length so that the tow vehicle also fits inside, just to make it easier to get everything hooked up and loaded while still protected from the weather. I don't know what kind of services you have run out to the barn already, but turning the area into a wash bay might save you additional time later - tile the walls, put in a drain pump system to the septic field etc. There may be an advantage to putting RV doors on either end - either for pull through or access.