Welcome to the forums! Glad to have you with us! It is generally held that adding significant weight to the rear bumper of an Airstream is a very poor idea and will ultimately lead to shell separation issues down the road...this applies to bike racks and certainly the weight of two generators. I would hesitate to do it. You can search the Forums for quite a bit if discussion on this topic...
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Travel Log: AZBambi...On the Road Again