Moosetags, I've considered the same, and corresponded with a certain Airstream customizer in CO that has done such. I've deferred the project, as we've discovered that we very rarely use the generator - solar keeps our batteries charged very nicely. The tongue weight issue can be managed. Switch to two smaller, lighter, fiberglass propane tanks. Build a lightweight support frame to carry a single generator between the tanks. (My Yamaha 2400is generator runs my 13.5 Dometic A/C just fine, at only 80 lbs.) A stainless cover similar to the existing propane tank cover would look fine. If additional weight reduction were needed, you might move the spare tire to your tow vehicle.
I also considered a mount on the rear of the trailer. My generator would add no more weight to the rear frame than does the loaded Fiamma bike rack offered by Airstream.
Running the generator in the transport position is a different matter. Exhaust routing, ventilation, sound and vibration dampening are issues that would require serious design consideration. I haven't found a design for a suitable safe enclosure for these portable generators.
Joe & Joan Donnaway