Grey tank to black tank crossover

I'm going to need to take the underbelly insulation off to see exactly what's going on under the coach.

I have always found gray fills first as you probley have. I like your idea if you have no pans to run gray out to the ground. And who does that aside from boon dockers? Great idea. Previous poster was great. Thanks for the post

My two tanks are on the same level and close together with a common drain. I use a cap with a hose bib on it so after I open both valves and equalize the tank levels...several times if necessary I'm able to drain the half gallon of stuff left between the valves before removing the cap. I have a hose valve on the hose bib and I put the drain hose up to it, open the valve, drain the half gallon and then remove the cap and put the hose on to finish dumping.



PS:: for those that take forever to dump with all the steps they take, in 50 years of doing this I do not use gloves. Yes, taking 10 minutes to dump is one of my pet peeves.

Some have posted in the past about adding a 12V pump via an additional line to the grey tank waste line before the waste valve and attaching the outflow to the vent stack for the black tank.

Old-Biscuit wrote:

About $25 for twist on

IF you do that, be sure to test it first using (mostly) fresh water.
I got a "see through" elbow for mine and the seal leaks at one end.
YUCK....nasty surprise.

jerseyjim wrote:
Just courious....why would you want to do that?

The tank that fills up first can be balanced with the lower tank to extend the time between dumping. That can mean an extra couple of days or more when boondocking for instance. I have the same third valve setup that Old-Biscuit mentioned.

Just courious....why would you want to do that?

DO all three tanks drain thru a 'common header' to one outlet?

If YES........
THen simple mod
Add an extra valve in horizontal run of drain header.
With it closed......
You can open grey tank and black tank drain valves and let them equalize

All three valves open.......114 gal WASTE capacity (14 gals more then fresh water tank capacity or 4 gals more then fresh water capacity ---fresh water tank and water heater)

I have done this mod on several of my trailers ....waste is waste


If not wanting to do the permanent valve install (3" drain valve and 2 flanges) there is a 'twist on' drain valve that installs right on end of drain line. Remove end cap, twist valve on.

I prefer the permanent....it isn't hanging on end of drain pipe adding extra weight, have to remove/stow for travel otherwise it is bouncing and could crack drain line or 'teets' that hold it in place.


About $25 for twist on



About $15 for glue in place