If you've been boon docking, you have basically just moved the water and the weight from the fresh water tank to the waste tanks for a net change of zero. Unless, of course, you've gotten buckets full from the creek with which to flush the commode . . . .
And yes, they are designed to carry all the tanks full, although as @bdpreece mentioned, if you go up or down very steep grades, it is possible to see some "unwanted" spillage. But that would take a pretty steep hill.
Commercial as well as state and national park campgrounds will usually let you dump in their dump stations, although they often do charge a small fee. Worst I've ever seen was $10, usually around here it is $5 or so. There are also free or nearly free dump stations at some rest areas in some states, as well as at some truck stops. I've seen them at Flying J's and Pilot stations, for example, as well as a few Walmarts, RV dealers, and camping stores.
I personally prefer to dump before leaving if the campground has a dump station simply because every pound you drag around costs fuel.
