That time of the year temps should be good. I've camped at Valley of Fire just north of LV. They have 2 campgrounds one is pretty wide open and the other has some secluded spots to pitch a tent. Boulder Beach (Lake Mead) also has a campground with some good spots. Either one of these can get full around the weekend. Southern Utah area near St. George has some good dirt roads to explore as well, just a couple of hours from Vegas. Death Valley is within a few hours as well.
Supercross is held at Sam Boyd. Sam's Town, Eastside Cannery and Sunset Station are closest casino hotel options. I've stayed at Hampton Inn & Suites Las Vegas-Henderson and there is a Holiday Inn Express & Suites Henderson both on Astaire Drive in Henderson. Close to restaurants and a short drive to stadium.