A defective check valve will cause a leak at that point. The only solution I know of is to replace the valve. I did that on my 2004 Designer, and everything has worked fine since. The main reason for opening the valve first is to prevent build up of pressure, which would make a mess. I open the valve and immediately also turn on the hose to the flusher. After the flow (as seen through a clear fitting) diminishes, I shut the valve, wait about 2 minutes with the flusher still going, then shut off the flusher and open the valve. When only a trickle is coming out, close the valve and I'm done.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.