Was out this past weekend in our 21 G-Series, and our sink backed up on the second day, so I took the plumbing apart under the sink and didn't find any sort of clog. Being that we were in the desert and hadn't been using the sink for more than a day I figured it was safe to crack open the grey tank to see if for some reason maybe the tank was still partially full from our previous trip or something? I cracked the grey and only about 2-3 gallons came out. We tried using a small Milwaukee leaf blower to blow air in the plumbing from the sink, tried a small piece of wire to try and see if we could find a clog at the sink end and the drain side, but we were unable to sort anything out and at this point the sink is still backing up! Really lost with this and any help is much appreciated!