IMHO, a bad idea... There is a huge amount of motion on the back end of a TT and the bumper is not designed to take the stress of the weight of bicycles cantilevered off of it.
As an example; I have a frame mounted hitch that I put additional reinforcement on...my Thule T2 tray type rack actually suffered metal fatigue and failure (happened on I40 around Oklahoma City; has to be the worst stretch of highway in the nation) on the upright that stabilizes the bikes (not weight bearing). I had to reinforce the upright with a welded on 1/8" steel plate to repair it.
CruiserRV makes a frame mounted, bolt on, receiver hitch for their TTs and I suggest going that route or having a good welding shop weld a hitch to the frame. I went the CruiserRV hitch route...it would have been easier to have it mounted at the factory while they were building my TT (the tanks are in the way of the bolt holes; fortunately I have small hands). I cut the flanges off about 2" and drilled new holes to mate with the frame's holes. This brought the hitch back a bit and up closer to the bumper and reduced the "lever" length of the hitch sticking out. I then used threaded rods and some drilled metal strips to make four Xs. Sandwiched the hitch between the two bottom Xs good and tight and then the bumper between the two upper Xs, also good and tight and then ran nuts up and down, in between the Xs to lock the hitch and the bumper in their relative positions for additional stabilization. The bumper offers stability, but, doesn't take any weight...the weight and torque is born by the frame (also substituted grade 8 bolts for the grade 5s that came with the receiver for added strength). I even went so far as to splice into the taillight wiring under the trailer and installed a 4 pin trailer hook for auxiliary tail lights that I mounted on my T2. The bikes cover the TTs taillights to some extent at some distances and angles from the back...the additional lights on the bike rack ensure visibility of brakes and turn signals. The T2 rack also has a 2" adapter, so, once set up at a CG, it is a simple matter of transferring the rack from the back of the TT to the back of the Jeep and we are good to go out exploring local bike paths.
My recommendation, if you are handy with tools, get the CruiserRV bolt on frame mounted hitch, it is a tight fit (actually had to "jack" it into position between the frame rails), but, there is enough room between the black tank and frame to get the bolts in; or... Go to a good welding shop and have them weld one onto the frame.
To be honest, I didn't really investigate, as to whether the unit over the tanks would be adaptable to using on the hitch of the Jeep once we got to a campsite. Quite often, we'll go somewhere with sightseeing in mind and then later look for bike paths in the area. Carrying two racks wouldn't cut it and a big disassembly and reassembly in the transition from over the tank to on the back of the vehicle wouldn't work well either.
