Just had to deal with a black tank rinse issue on a factory installed system. I learned a lot in doing the work myself. As it turns out the hose connection goes way up a pex pipe to a spot by the master bath vanity that is very hard to get to. It enters a B&B 517 vacuum break backflow device, then exits back down to the sprayer in the tank. When you put water in the hose connection it closes the vacuum break and opens the backflow valve and water (normally) goes to the sprayer. It also makes a very loud vibration noise. When you disconnect the hose when you are done, the backflow valve closes, the vacuum break opens and the water up to the B&B 517 pours out of the hose connection. This is normal operation.
In our case it worked 5 times right and then the 6th, 7th, and 8th time the vacuum break didn't close and water came dripping down into the cargo bay. Contacted B&B, they sent a new 517 and an envelope to send back the defective one.
My guess is that the water from the backflow valve to the sprayer stays in the pipe until the next use, unless some air can leak into the pipe from the sprayer end. Kind of like putting your finger over the end of a straw and pulling it out of a glass of water.
2012 GMC 3500 Duramax Crew Cab, SuperGlide 20k
2014 Cedar Creek 36B4, LP Onan, Level-Up, Trail-Air pin, electric fridge