I've got Yakima racks on my second truck/camper shell combo (as well as three other vehicles). I've bolted the "top loader" to fiberglass shells in both cases and they have been bombproof, but placement and how you attach them matter. Put the top loaders near the corners ("knuckles") and back the loaders up with big washers/plates. I frequently carry several boats (canoes, kayaks, c-1's, as many as 5 boats at once) and have toted a good bit of lumber (up to 4 sheets 1/2" plywood) - easily 200#.

I've got a few pictures here - scroll down to see what I did with the shell and what I would *not* do again on the cab :

http://bookwoman.net/scott/truck/


Thule has similar, but the bars are *maybe* less stout - I've just got a lot invested in Yakima accessories.