The Airstream Frame has been designed to carry the trailer and the personal stuff we put in it. Adding a 250 LB weight to the rear bumper will overload the frame and can cause it to fail, not
to mention the way it will adversely affect the tow ability of the trailer and tongue weight.
You may want to consider a different way to transport the motorcycle. I have seen some custom hitch extensions that would go between the trailer and tow vehicle. But then you have to look at total hitch weight and the capabilities of your tow vehicle. Not to say some people do put bikes and such on the back of an Airstream but the mounting of such items can cause frame problems>
Just my 2 cents worth. I am sure that a more reasoned response will come from some of our more senior members that have more experience.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato