We've had our 2019 North Trail 27RBDS for about 2 yrs and have thoroughly enjoyed it. However, we have never been able to hookup a water hose to the Black tank flush and had the water going thru the hose actually go into the black tank. I've tried it on 3 occasions and nothing. The black tank sewer hose is on and have already emptied 80% of the tank. But, either looking thru clear elbox at end of hose and seeing no water come out, or looking down through the top of toilet, there simply is no water entering the tank from the black tank flush. I've read thru the Water Systems Guide provided by Heartland to no avail. I thought maybe there was a shutoff valve somewhere stopping the black tank flush water from going into the tank, but I see no evidence of that. Thoughts? We bought the camper new and tried the tank flush within a few short weekend trips and have yet to get it to work. Any assistance is greatly appreciated.