To wit they've done everything. Bus derby, RV derby, trailer derby ... it always ends in wonderful amounts of destruction and spectacular crashes.
Oh, and it turns out RVs disintegrate rapidly. I learned that it's not the carnage, per se, but the becoming of carnage that is entertaining. The show was over too quickly. In fact, it ended when one of them caught fire shortly after they lost their radiator. If there were more vehicles then that wouldn't have meant the end of the show. Maybe buses--well built buses--would have been more fun.
OTOH, the only demolition derby I'd been to previously was last year's derby at the Alameda County Fair, which was all cars. So probably I should be careful about generalizing.
EDIT: The 2017 fair had an RV demolition derby with 5 RVs in the arena: https://www.youtube.com/watch?v=I-Al1TISq_s