There were some empty sites, but I don't know whether they were sold or not. Certainly not all of them were sold because I got my site (without a reservation). I was actually surprised at how full the campground was on a Thursday night, but then again it is prime camping season in New England.
I would venture that most people would find a cheaper place to dump if they needed to do that on a regular basis. While emptying the tanks was what made me decide to go camping, I was of course also in it for the camping part, too.
I hadn't thought about hazmat disposal problems. It would seem to me that it might be obvious something is up when someone drives up and pours stuff down the hole in the ground rather than connecting a hose up to their RV. I don't think I would want to put a bunch of paint thinner or whatever down my gray/black tanks....but then again, successful criminals have to be pretty clever and devious.