Our check valve for the black tank rinser is behind the mirror on the wall between our dinette and the shower. That's where we can get at the hot/cold knobs in the shower, too.
Ours got stopped up and we replaced it but that didn't solve the problem.
The problem was that the sprayer inside the black tank was also stopped up.
We could see little bits of grit/sand in the check valve when we took it out. (MAKE SURE YOU HAVE ALL THE WATER TURNED OFF going to the rig...and the pump turned off when you work on this stuff.)
So lesson learned... put a sediment filter on your hoses no matter if they are going into your fresh water tank or onto the black tank sprayer.