I have a brass hose adapter with a ball valve. The key is the process. So, when I'm breaking camp, and after the tanks have been drained, I turn the water off, then unhook the fresh water hose from the trailer, and connect it to the adapter with its ball valve closed. Then, I open the hose bib and pressure the water hose, THEN I open the ball valve to begin the black tank cleaning with an open drain valve. I let this run for 3-5 minutes.
Then close the ball valve, close the hose bib, then remove the water hose. Yes it's still pressured. I break the hose connection at the campsite hose bib first to drop the pressure.
One of the campgrounds we frequent does not have sewer hookups. They have dump stations. These dump stations have a dedicated overhead fresh water hose that can be pulled down for flushing the black tanks. (I wear gloves) You have to be careful with water usage at this campground, but otherwise it's a fantastic place. They do have clean bathhouses, and although it's a little more of a pain, I really don't mind. It's just a short bike ride away to a hot shower, and saves a lot of grey tank capacity. Sorry for getting off topic...
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16