Rear end separation is a known Airstream "fault", and is more prevalent in longer A.S.'s with rear bathrooms. Some were fixed previously with a less than stellar solution, as ours was, but now it's FIXED! Not difficult but takes some investigation on the forums with different ways to do it and some labor to get everything level and fastened together correctly again.
Our problem was that the rear floor had rotted out. The trailer is put together bottom up: frame, floor, C-channel and trailer walls on top of the floor. The floor being plywood, rots due to a couple of other inherent problems such as not waterproofing correctly when originally built, or lack of maintenance over many years. The solution for us was to replace the rotted floor, and beef up the connection between the rear frame and shell of the trailer. We used angle aluminum to do this along the rear C-channel. It's very solid now. It was not expensive in materials but took a little time and finnesing. We also gutted our trailer so we had very good access to all of the problem areas to fix. If you can do the work yourself, you can save yourself a bundle!