It is definitely possible to mount a RTT on a set of Yakima or Thule bars. We have a lot of customers that do just that. And it is really easy to mount and dismount this way too. Yakima makes a really cool gizmo called a Universal Snar. It is a nylon reinforced clamp designed for use on either Yakima or Thule bars. It clamps around the bar, so you don't have to do any drilling. The hole fits an 8mm bolt. The DIY installation kits included with the Eezi-Awn have 8mm bolts just the right length to go through this clamp and hold the tent to the bar. Removal is just 4 nuts and lift off.
I prefer the Thule bars to the Yakima bars because Thule bars are flat. That puts flat bar to flat mounting extrusion on the tent bottom. The round Yakima bars are fine but over time I am concerned they will flatten the extrusion that holds the rectangular mounting washers that slide in the extrusion on the tent bottom. If this extrusion is compressed, the washer don't slide. That's bad.
Please make sure that you do check the load rating of your shell. Keep in mind that the rating is a dynamic weight, or a weight in motion. The static or stationary weight will be much higher. When stationary, a good portion of the weight on the roof will transfer to the suspension. Eezi-Awn has been installing roof top tents on fiberglass shells for several decades now, with very few problems. The problems are usually the owner, not the truck.
Another suggestion is if you choose to go with a track based mounting, mount the track as close to the vertical edge of the shell as possible. This takes the stress off the roof and puts it more vertically to the suspension. A track mount is preferable to the side gutter bolt on brackets, because the track can assist in dispersing the weight of the load over a larger area. The bolt on gutter brackets are very nice for light loads, but putting a larger load transfers all that weight to a series of bolts drilled through a fiberglass sheet. The vertical stress on those 1/4" or 3/8" round bars through that thin sheet will cause trouble down the road with cracking and pull through.
Hope that made sense.
Cheers,