If it's the round light with 9 LED's like I'm thinking it is, it should be pretty lightweight.
If you have any kind of storage door (with screws in the frame) or a window with screws in the frame, drill some small holes in the bracket and use an existing screw(s) to mount the light. LED's use minimal current, just wire it into the backup lights.
At any rate, I'd avoid the glue thing. It'll weather and fail sooner or later.
You might be able to make a wrap-bracket with thin aluminum to go around the rear bumper. I'd think mounting the light higher up would work better though.
I went a little extreme in the back of the coach, I used a set of HID's with a Euro beam pattern. They create daylight for 500 feet behind the rig for the backup camera. Legal? Doubt it. :B