It depends on the gauge of the tube used for the bumper, and how its attached to the frame rails. Lots of variables. About the only thing that is kind of common is if its a Lippert frame most likely a bad idea without a lot of added reinforcement and there is a 100% chance your Grand Design has one.
All of that said I see plenty of folks with bikes hanging off of rear bumpers and have yet to see a pile of them on the road other than pictures posted in RV/camping related forums, and its usually the same picture that has been posted thousands of times for 15 years LOL. Heck I seen a Grand Design TT picture where the entire rear wall fell off on the way home from the PDI.