I attach to the faucet a brass backflow prevention valve that I got at Lowe's for a few dollars. To that I hook my dedicated black hose for the flushing. I'm fairly certain that my black tank flush system has its own backflow prevention, but I use the additional one to be safe. Before and after this operation, both the faucet and the brass valve get sprayed with a sanitizing solution.
2013 Surveyor SP220
2010 Silverado LT 5.3 V8
The world is a great book, of which those who never stir from home
read only a page. - St. Augustine