I've used that site also... it's great. There is also an iPhone/iPad app (it may exist for Droid also, but I'm not sure) called AllStays Camp and RV. It's an excellent little app that costs about 5 bucks and has an incredibly thorough listing of campgrounds (public and private). It also has add-on apps, one of which being dump stations. I've used it quite a bit for Oregon, and a bit for Washington, Northern California and Western Nevada, but when I scroll around on the map, it shows a lot of listings for the rest of the US and even Canada. It has everything from small two-site forest service campgrounds to the huge RV resorts. For the dump stations, it lists water treatment facilities (most of which have free dumps), rest areas, truck stops, gas stations, RV dealers and many other places I had no idea had dump stations. For us, it's been well worth the money since most campgrounds we stay at don't have a dump station. I'm able to check the map and usually find a free or cheap dump station on the way home.
Marila