I either dump at the camp ground or usually dump at the university where we tailgate 6 times a year. The camping is strictly boondocking, but the university does have a dump station we can use. That's always a "quickie" deal. Can't take time to flush out the black tanks. But that doesn't usually matter. We'll be using the trailer again the following week.
If I need to really flush out the black tank because we will not be using the TT for a while, or we've boondocked somewhere dumping is not available, we will drop by a local campground. So far, they let us dump for free. Of course, anytime I need propane, I go there to buy it.