I rented a cargo van twice from Enterprise. I ended up buying one because I cannot spend 2 days cleaning again. Seriously. The cleaning is not fun. That being said, you may want to try Enterprise. My first year I booked far in advance with AVIS and they magically didn't have a van when I went to pick one up. So I got one last minute at Enterprise. It went so well with Enterprise that I reserved one again the second year. They asked me, no less than seven times, if I was going to Burning Man, to which I replied "of course not!" I wasn't charged any cleaning fees, but I didn't want to chance it again.
I drive from Santa Cruz (90 miles south of San Fran), and gas runs me quite a bit, but whatever. If you end up with a rented cargo van, Be sure to ready to spend at least a day cleaning. Amour-all Leather Cleaning Wipes are your friend.
In dust we trust.