I put your departure city in the subject line just in case it'd help you get any nibbles. You might also post on: http://rideshare.burningman.com
You're probably gonna make someone else's day.
If you've got 4 or more people in there already, don't sweat it too hard . . . maybe you know this already, that's when it starts feeling a bit crowded no matter how many the RV is rated for.
Best of luck!