[davidpriolo]

I can't say which valve to turn, I don't have your plumbing diagram. It should show in the owners manual. I have had a similar problem caused by the vent being plugged. You must let air in to the tank order to suck waste out. Use a dental pick to remove the fire screen in the vent. A powerful vacuum suction will actually collapse your waste tank if the vent is plugged.  Now that's a real mess.  That screen is only needed on fuel vents and corrodes up badly on waste tank vents.  I also removed the screen on my water tank vent. No more slow fills. While you are at it, clean your fuel tank screens for a faster fill without a blow back. I spray in WD40.  Vent maintenance should be on every body's to-do list. Bugs love to live there and plug them up.
When you unscrew your fill caps listen for a hiss. If you hear one you have a vent problem.
Capt. George

thanks guys,  Ill try the engine room  valve first