Gray tank problem

Turns out it was the air admittance valve under the kitchen sink. I just replaced this not too long ago. At least they're not expensive.
Thanks again for all your help.

Thanks for the replies. Looks like I have a project for tomorrow.

Before I dump... I put water into the kitchen and bathroom sinks - enough to form a seal. (I've even put some water in the tub on occasion but that doesn't seem to need it like the sinks do in our RV.) I make sure there's water in the toilet bowl, too. THEN pull the black valve. when you are done flushing the black tank, Shut the black valve and then open the gray tank valve.

You should hear and feel a bit of a suction going on in your roof vent when the black valve is pulled. The fuller our black tank is, the bigger the suction feels.) It will acdtually need to "burp" to get more air... so if your flow stops and you hear a gurgling going on in that vent, just wait a bit for it to draw air down into the tank. If it just won't draw down air, CAREFULLY flush the toilet to let air in that way. If you open it too wide, you'll get back splash as the water in the toilet pipe will have an air bubble going down... so just open the toilet a bit to let the water out of the bowl go down the pipe..and some air go down with it. (This is going to mean that your vent pipe is plugged - and you will need to figure out why and fix that.)

Then let the water drain out of the sinks (and the tub, too, if you have water in the tub). That will refill your P-traps. The suction will pull the water out of the P-traps when you flush and if you don't put water in there, smells from the tanks could come up into the rig...

As stated, check your roof vent, it might be blocked, thus it is sucking the air from your sink drains when you drain your holding tank.

Improper venting. Do you have a roof vent?

Your new "anti siphon devices" are air admittance valves. If you are getting an odor under the sinks, the new valves may be defective. Here's how I'd approach it.

1. Fill the gray tank up enough to run a test. 2. Make sure you run enough water down the sink drains to verify that the p-trap is full. 3. Drain the gray tank. 4. Loosen the nut on the p-trap and verify that the trap still has water in it.

If the trap has been drained, replace the AAV. If the trap has water in it, you still have an odor under the sink, remove the AAV and put some thread sealing tape on the threads of the AAV to make sure gasses aren't bypassing the AAV through the threads of the device. You might also try using a gray water odor controlling additive that you can purchase in most RV shops such as Camping World.

If all else fails, try replacing the AAVs with another brand.

Sounds to me that your gray tank vent isn't working as designed.