I think Pat's idea of the VHB (very high bond) tape on a conventional track is a good one.
Is the U-haul's outer shell is much thinner (or any thinner?) than, say, a Scamp
shell. If not, well, many people install an awning track on just one shell of those, so there may not be any particular need to tie the inner and outer shell together since you are not using conventional fasteners (which would try to draw the two shells together). Either rivets or VHB would not do that, and would not require access to the back side.
Unless you had a major awning, I wouldn't think it would "tear" the outer shell (but correct me if I'm wrong since I don't have a U-haul
to go poke at), and with just the VHB, there would be no holes. I don't know if it would hold an awning that sat on the side of the trailer all the time (it very well might though), but there are many that you take out of the track between uses. Putting one in the track (if you went that route) would probably be less work than putting up a First Up (not that there's anything wrong with a FU; just saying that to compare the work if you "reload" the track each time.
Once the track was in place, one might find it easily strong enough to hold a bag awning up there enroute.
If you felt you had to tie the shells together, then I would be thinking of making some thickened epoxy annuli between the shells. But I still think outer shell only might be viable. With rivets or VHB tape (the tape does sound appealing).
One note on the tape: Although it seems unbelievable, 30 years down the line, there is usually some contamination from mold-release wax on the outside of the trailer (gelcoat). Solvent and careful wiping will remove it (keep using fresh side of a towel, with no re-wiping, etc.) Interlux Solvent Wash 202 is one product, but I'm sure there are others. Acetone might work in a pinch. You would want to do this before sanding, or otherwise you can just grind the wax in.
By the way, if you make your own awning, there is some plastic "tape" that is basically the "cord" that goes through the awning track, with a flap attached to it. You sew the flap onto the cloth, and not around the "cord." That's kind of nice because it used to be that the fabric would always wear off where it wrapped around the rope (that you would sew into the fabric in ye olden dayes).
(I haven't sewn awnings for trailers, but there are a few different canvas awning-type things on boats that use the same ideas.)
PS: Whew, I guess my coffee kicked in!