You have a really nice shop there already. I like the idea of adding on to it. Why are you messing around with this old shed when you could easily have something a lot nicer next to your shop building?
For your RV, you do not have to have the lean-to start at the top of the wall and under the existing roof. You can easily make the lean-to wall the same height as the building and bring the roof up to the middle of the existing roof, or all they way up to the peak of the roof. Neither are very hard to do. This will give you plenty of clearance above the RV to span the distance that you need with trusses, or 2x12's, depending on how wide you want to make the building.
Before spending a lot of time and effort on moving the shed, I would decide on what it is worth as it is and what it will cost to make it like you want it after you have put the time into moving it. Figure out that dollar amount. Then figure out what it would cost you to build a brand new shed. My guess is that when it's all said and done, you are not saving enough to make it worthwhile by moving the existing shed, and then fixing it up after you have moved it. Then figure out what it would cost to add a lean-to onto the really nice building that you have, and see if that's where you will have the best return on your money.