I would be very cautious mounting anything to the rear bumpers of trailers I have looked at. I would most definitely extend extra supports down to the main frame.
The rear of the trailer is probably the most violent place to mount a generator to. There will be alot of bumps and movement in this location heading down the roads.
Google search shows some great shots of beefing up the area under the trailer supporting the rear bumper.
In my case my bumper is directly connected to the frame and the rear of my trailer wall so it should be a pretty solid setup.
We looked into having an additional carriage points in this area as well. As you can see I really don't have alot of room with the rear tent bed having to be pulled out when setup at the camp site. A generator running under the tent bed is probably not a good idea as well as for as fumes are concerned. Finally just left my spare tire mount all alone and elected to not use the rear bumper area for anything.
As far as mounting a generator to my trailer bumper or any other outside area was also thought about as probably a mistake for our style of camping. The generators are very expensive items and the thought of leaving them unattended out in the open would worry me to death. Most all of our camping is off the power grid with alot of times the only camper sitting on the creek bed very prone to thief. After thinking about it I just could not do it. It is bad enough to worry about the batteries and propane tanks on the trailer tongue area.
My 2KW Honda EU2000i sits secured in the tail gate corner of my covered and locked truck bed. It is completely out-of-sight until I use it where I will slide it out onto the tail gate when running. My generator goes where I go... Out-of-sight equates to Out-of-mind in my way of thinking.