First post ever, so here goes!! I just purchased a 2014 Columbus 320RS. I have spent a month trying to figure it out. Here is what I know about the tank drains.
There are 2 grey tanks and 1 black tank. They all drain out of the black pipe on the drivers side. This pipe should have a twist on cap on it. There is a "T" pull handle to the right of this black pipe. It should always be pushed in!! That is, unless you are working to drain the tanks.
Inside the "water closet", on the bottom, are 2 "T" handles, one, in my case, is labeled "GALLEY TANK" and "BLACK TANK".
Now for draining the tanks: 1) carefully remove the black pipe cap and connect an appropriate drain hose. Make sure the other end is in some type of septic tank. 2) you want to drain the black tank first. Pull the T handle for the Black tank from the water closet. I have a hose connector in the water closet labeled "BLACK TANK FLUSH". Connect a hose to this, turn on the water, and it will rinse (somewhat) the black tank. Push in the T handle for the black tank. 3) The GALLEY TANK T handle drains the bathroom grey tank, not the kitchen!! Pull that handle to drain the bathroom. 4) Now to the T handle to the right of the black pipe. This drains the kitchen tank!! Pull the T, and you should have all your used water tanks drained. Push all T's in, put the cap back on the black pipe, and you are ready to go!!