Are they able to drive the sukkahmobile without the schach falling off? (If you allow cable-ties you're all set, but if not ...)
From looking at a couple of them around my neighborhood, they all seem to use some kind of bamboo mats. Some (maybe most) of them take off the sechach while they're driving, and put it on when they reach their destination; others seem to keep it anchored in place through judicious use of 2x4s (with notches carved in them so they'll stay atop the walls).
And after all, the ones I've seen are designed for driving around in New York traffic, so they're necessarily going slowly.