Which pipe is which????

the vent for my black is pretty much in a straight line up from the toilet and the grey is above the shower and the galley tank vent is a bit more forward... at least on our trailer.

Thanks so much for all the good advice. I've only owned the RV for a few weeks, so everything is trial and error.

Before dumping anything down the vent pipe, make darn sure it IS a sewer vent! Some RV's have a second vent cap that serves the "attic" air space between the ceiling and roof. Not a great place to be dumping in water... :)

Another option would be to have someone flush the toilet and put your ear near each vent to see which has a running water sound. If you can't get close enough on the roof to the vent, you could put a length of hose into the vent and listen to the end of it.

Could also dump some food dye down the vent to see which outlet it comes out of.

RoyB wrote:
Probably the only sure way would be go up on top and take a big whiff of it...

You can always call the manf with your VIN number handy and ask them.

Roy Ken

And the one that stinks LEAST, more often than not, is the black tank.


Here is a positive way to do it....

Get one of those clear extensions so you can see the dump and put it in line with your hose.. I like to put it at the RV end.

Dump tanks and CLOSE VALVES.

now, take a couple gallons (or a hose) up on the roof with you and run/pour some water into ONE of the vents, Just one, not both.

Go back down and open first the BLACK tank, if it dumps a bunch of mostly clear water.. You have a lead.

Then open the gray, if it dumps nothing, You have your answer

On the other hand if the black tank dribbles and the gray gushes.. Well, then you have the other answer.

Close the gray tank drain valve. Open the black tank drain valve. Stick a water hose in the top of the vent pipe and turn it on slow. If water comes out the drain you've found the vent for the black tank.
BTW, the vent pipes are primarily installed to prevent a vacuum being formed when water runs down the drain. Without the vent, the vacuum would pull the water out of the p-trap then allowing sewer gases to come up through the sink or toilet. Thus the use of air admittance valves at sinks difficult to reach with a vent.

Run a good slurp of bleach down the kitchen sink before doing the sniff test. The contrast should be obvious, even with a new camper.

PS- Usually good practice to avoid putting weight on roof unless absolutely necessary.

Thanks for all the advice. The pipes are both directly above my bathroom, and look exactly the same. I guess I'll try to sniff test, or buy another vent.
Thanks again.

Moved from Forum Technical Support

harold1946 wrote:
Put one on each vent. That will cover you when water in the p-traps evaporate.
Grey water can become just as offensive as the black.

X2