Many gas stations on the interstates have dump stations as a way to attract RV's with their thirsty engines and large fuel tanks. Many campgrounds and all private RV parks have dump stations.
As part of my trip planning I check whether or not a camping spot has a dump station. 90% of the time one is available. When we are at a place with a dump station we use it regardless of how much is in the waste tanks so we have the full tanks' capacity available. Fresh water we get at our destination rather than haul around the extra weight.