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The throat of the drain must be plugged up. You need to get a wand or snake and put down there and get it unplugged. OR the tank is full and needs to be drained. Stubborn clogs can be broken up using Happy Camper Extreme Cleaner. Great product! If your local dealer doesn't stock it you can get it directly from the manufacturer in Medford, Oregon. See the attached link. Please get back to me if we need to dig deeper into this (no pun intended) as all follow ups are included in our fee. https://www.5starhappycamper.com/
Just checking back in to see if you got this resolved or do you still need some additional input from me here?