I don't like the 'twist on' valves because I can't leave it attached all the time.
So I installed another permanent valve on the horizontal run of drain line just before elbow where it turns out to side of rig (I've done this on the last 3 RV's...maybe 4).
I use it as a stop gap measure for leaky waste tank valves, to equalize the two grey tanks and to back flush grey water into black tank.
Simple modification that helps eliminate problems.
2 glue on flanges, 1 valve and about 1/2 hr. of my time-----priceless.
