Our '88 Suncruiser had the combination wye valves for the black and grey tanks, 3" valve on the black tank, 1 1/2" (?) on the grey tank. It took a long time for the grey tank to drain, and was frustrating because the line from the tank to the wye was 3".
So, I removed the entire assembly and replaced it with 3 3" valves and a Tee. The 3rd valve was on the leg of the Tee.
This afforded me 3 things I found useful.
The grey tank dumped faster.
After a while I figured out a better dumping procedure. As there were usually just 2 of us in the coach, the grey tank filled up more rapidly than the black tank. So when dumping...
Open the leg valve, open the black valve, dump the black tank.
Close the leg valve, open the grey valve, and use grey water to back flush the black tank.
Close grey, open black and dump the back flush water.
Repeat for as much grey water as is in the tank, 2 or 3 times or so.
Finish dumping.
The last advantage is that, in a pinch, we could tie the 2 tanks together to lengthen the stay time when boondocking. I only had to do this a couple of times, but it really helped.
Cut to our Sunstar. I was gratified to see that Winnebago had switched to 3" lines all around. When we were shopping for new hoses and such, I was also looking for the parts to build my 3 valve dump manifold and was doubly gratified to find a Valterra 3" no-tools "repair" valve that has the bayonet fittings on each side It fit OK in the space available on the Sunstar dump hose fitting. So now I have my 3 way manifold again.
It's been a while since I've had to buy a dump hose, We purchased a Rhino hose with the clear elbow at the drop end. It's been interesting to observe how much more "stuff" (if you know what I mean
) flushes out of the black tank during my back flush routine...
Here's a pic of the Valterra in place.