On top of the black water tank is a spray valve, you have to take that valve off and clean it, it is more than likely clogged up. By taking the valve off and cleaning it out you should be able to have free flow of water. I do not believe there is a check valve or back flow valve therefore I believe your best option is the spray valve.
Good Luck
Raymond W. Flischel
2007 Mandalay 40' DP