Also, I saw somewhere that they will have shuttle service along the main road. So, you should be able too camp in dispersed camping and catch the shuttle back and forth.
Mentioned here:
http://www.overlandexpo.com/app/
UPDATE—MAY 17: Now offering SHUTTLE service from nearby camping areas to / from Overland Expo.
Although we are no longer selling Weekend Passes with Camping on-site, you can still purchase a Weekend Pass and camp nearby. There is a lot of dispersed camping areas in the surrounding forest. Check
www.overlandexpo.com/flagstaff/#camping for map and recommendations. Please note that if organized campgrounds are full, there is plenty of dispersed camping along forest roads. Utilize a topo map or your mapping app to find these roads, which do not show up on the map on the web page above.
The shuttle will be running from 7 am – 6 pm, in a loop on FR90 / Mormon Lake Road and Lake Mary Road. To catch the shuttle, which will say Overland Expo on it, you must be visibly standing next to the main road (the shuttle will not be going into campgrounds or on dirt roads) and flag the driver when you see it. The shuttle will run continuously and not at exact times, so please be flexible with your schedule.
If you are driving in and parking on-site is full, there is additional parking along Mormon Lake Road. Feel free to catch the shuttle to the entrance of the event.