thumbelina, I can't count on how many problems I had to resolve during my limited bathroom remodel on my old Trade Wind. I replaced about 4 feet of subfloor, but the shell stayed on. Shell off it the biggest aspect of rebuilding an old Airstream. So I can imagine your anxiety. You are right just to back off a bit and think about this task a bit more.
And you are right not to use the building roof as a hoisting point, unless you get expressed permission from the building owner. Most roof structures and not designed for heavy loads in small areas.
There are just lots of folks who have used the jacks and saw horse (supports} method of lifting the shell. The overhead gantry and hoist method saves time and may put less stress on the shell. Inland Andy suggest the jacks and saw horse method. I would probably use that method myself as I don't have a gantry. Some then use their gantry (you need two of them) to lift the frame and flip it upside down to make frame repairs, insulate, belly pan, and axle installation easier.
Look for a refab thread that uses the jack up method. Here is one I'm following. The shell lift is in the first few posts. At least you can see a picture of how they did it.
PS I see Dingo Girl just logged on to your thread. I've been following her project also. She is very skilled and I bet willing to give you some guidance. And Beckybrille is very, very experienced in all things Airstream. They have done several of them. Lots of experienced folks on these Forums willing to answer questions. You have friends that have done what you are about to do.