There are two possible problems with putting your bikes on the back of the trailer: 1. The bumper is very thin and may fail, and 2. very few bike racks are rated for the dynamic loads of the back of a trailer (being "RV Rated" is NOT enough). The following thread discusses the bike racks in detail, including links to bike racks that are actually travel trailer rated (you will notice that they are much beefier than most):
https://www.forestriverforums.com/fo...rs-230393.html
I mounted the Ultra-Fab-35 on my Surveyor 243RBS. It is welded to the frame just forward of the bumper. I also mounted it on top of the frame instead of the bottom for easier access to the bike rack.
The tread above includes a statement from a bike rack manufacturer that explains the side-to-side motion of a trailer and metal fatigue that leads to so many failures.
One other note: Bike rack manufacturers don't advertise that their racks are NOT travel trailer rated. They often say on the web site they are "RV Rated", and when you get it out of the box there is a sticker on it that says it is NOT for use on travel trailers.