I'll add evidence for this since I was looking for this information back in the spring. Hopefully this will help the next person. You can easily make this work with two crossbars. I took a trip out west in June for two weeks and never had any issues. Fair warning, my wife and I combined definitely don't total 300 lbs, so I can't speak for more weight than that. I used two bars as a quick and cheap answer to needing to mount my RTT. Seems like there's never enough prep time. If I was going to do it "right", I would probably make a custom roof rack and have it be as low to the roof as possible with a third crossbar. Using the gutter mounts and bars left a lot of space under the tent, which i didn't like. It probably didn't help mileage, but then again I never expected it to be great. I also tried to keep the front mount as close to the b-pillar as I could get. Since I was loaded up with two-weeks worth of stuff, I also tried to keep the tent a little more forward since I couldn't even out the interior load at all.
Totally sturdy...