Just be very much aware that most trailer bumpers are very thin metal and not designed to support anything - mainly for looks I think... I would also be very weary of leaving my high dollar generator unattended under any circumstances unless they are out-of-sight. They will walk away on you no matter how secure you think they are mounted in the open on the back of your RV trailer....
I would think something to support 400 lbs are so would most definitely require tieing into the trailer main frame for support.
Even simply spare tire holders are sometimes too much for the trailer bumpers...
I found alot of images on google search by looking for 'RV trailer rear storage racks'
Roy Ken