Yup, I rented a trailer from them this year and had the same thing. My wife made the reservation and was told there would be $500 deposit, but when I showed up to pick it up (after coming in a couple of days before to have my truck Ok'd as the towing vehicle) then they hit me for the $2000 deposit.
I went a little ballistic myself as I have heard other stories about Burners getting screwed by rental companies. I was in a tough spot as I was picking it up the morning of leaving, and had a lot of stuff for my camp to load into it.
So I bitched. I had a long conversation with them. Basically, the extra deposit was from the service department, and they agreed it was crappy of the sales department (that takes the reservations) to not inform people up front. The reason for it is was the craptacular way that previous Burners have treated their stuff, bringing it back not just a little dusty, but seriously trashed. One guy was nice enough to leave his trash and mud encrusted bicycles in the RV one year. They did clearly write up and communicate how they expected the trailer to come back and I spent the time to talk with them about it.
As a result, my wife and I did a couple of things. We pulled out the supplied sheets and comfortor and used our own. We covered all the other fabric surfaces with old sheets, put down an old carpet inside the door and big piece of astroturf outside. We also were very good about closing windows and vents when the dust picked up.
Even with this, we still spent 8 hours cleaning the thing inside and out when we got back. We reserved an extra day just to make sure we would have time. And when I brought it back, I made sure the service manager knew I was a Burner and that it came back cleaner than when it left. I even showed them some things that broke during normal usage, and they did not try to charge me for them. They signed it off immediately and refunded my deposit promptly. No problem. She thanked me for working with them and was interested in my feedback to improve the process next year.
But another guy was looking at 15 hours times $75 per hour for how he brought his back. But hey, they made it clear what the deal was when I picked it up, so I really don't feel bad for him. If you don't think our community is capable of such behaviour, I guess you didn't see all the bags of obviously Burning Man trash overflowing the rest stop dumpsters along 80, or all the trash strewn along 447 from people who convinced themselves that one bungie on a glad bag was sufficient at 70mph.
The RV places really only have 2 ethical choices to deal with us. Either refuse to rent, or communicate their expectations and charge a deposit up front. That is much better then them hauling your ass into small claims after you bring back a trashed trailer.