Yakima Bike RackThis is the bike rack you need for the back of a trailer. You will notice it is a good solid rack made by Yakima. Most bike racks have a joint that folds allowing access to the back of a vehicle, but on this one it is fixed, it does not fold. This will not break with the bounce of the trailer. I have one on the back of my 5th wheel, bring up to 4 bikes, have traveled to Glacier from Seattle, never had a problem, only dirty bikes if we hit rain.
As everyone has said, do not use the bumper, that will go bad - period. I have a class 3 hitch welded to the frame of our trailer.
If you do this you can travel with good piece of mind.