There is a basic problem with mounting anything to the rear of an Airstream. The construction of the frame and shell does not have enough strength to do so successfully. The design uses the shell to hold up the frame somewhat, and extra weight can easily make it sag.
There is also an issue of stability--weight that far to the rear causes possible instability because it reduces tongue weight and can make the rear end too heavy.
There have been many documented cases of trailer sway caused by adding too much weight to the rear end. Best place for a generator is inside the Tow Vehicle, mounted over the A-frame, or simply carried in the AS forward of the axles...
I have a smallish (3200 Watt) dual fuel Champion Inverter generator--it lives in the bed of my pickup under a tonneau cover when in motion, and chained down when in use as far as I can get it from the AS.
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.