Just because someone has done it and someone built a rack ???
IMO, its not a good idea for YOU to alter a vehicle that is being used on a public highway. Carry it in the TV where it can't fall off.
Who would you possibly trust to build you a strong enough bumper to haul that much weight on the rear of a TT that has limited suspension with no shocks.
I would consult with a liability atty before attempting this. Explain to the atty that you want to alter an RV to be able to carry a 130 lb.portable generator on the outside of a TT by modifying the rear bumper and you plan to haul it on the highway.
Regarding NIST Techical Note 1666 : Also..... Ask about using a portable genny on that bumper rack within the minimum distance of a building (your RV). (carbon monoxide)
Ask who would be responsible if a serious accident occurred ?
I can tell you it, its not going to be the mfg of the 27 ft. 06 travel trailer.