Just My two cents on the topic of whether or not to do a shell off: It took me less than an afternoon and maybe $100 in lumber and hardware to build two wooden gantry frames to lift my shell. The work it takes to unfasten the shell from the frame is the same whether you try to replace the floor with the shell on or not. If I had to do it over again, I would have left the bellypan in place except around the edges, pulled the shell, then flipped the frame over with the gantries and worked the belly pan with the frame upside down. I reinstalled the bellypan this way, and have zero regrets, it made the job so much easier.
People look at the apparatus involved with lifting the shell and get intimidated, but I believe religiously this is the way to go. It just makes everything else you do so much easier. I welded on my frame, pained it, installed the subfloor, installed the insulation underneath, and put on a new bellypan, all while flipping the frameover and over as needed.
If limited space is a concern, I have a hard time imagining less space than what I have. I had my shell sitting on one half of my driveway, and the frame occupying a small patio slab that used to be home to a patio table and chairs. Of course this is with only a 21' trailer.
See the attached pic--shell on the driveway, gantry frames being positioned to flip and rotate the frame, frame upside down for insullation installation, grey tank fitting, axle installation, and bellypan install.