Here’s what you do if your gray tank is clogged.
Usually this is a new rig to someone and they just haven’t found the pull handle for that particular tank yet. There are often multiple gray tanks. Sometimes they’re called “gray”, sometimes they’re called “galley”. But more often than not, when someone has a new to them rig, there’s a pull handle they haven’t found yet. To find it, look up by the frame and around the area of the tires.
Other than that, drains get clogged. We are a bunch of Wookiees. If you have a traditional P-trap, just use one of those hair getter outter tools. If you have a waterless (aka Hepvo) trap, then you must remove that before putting a tool down or you risk tearing the diaphragm in the waterless trap.
Lastly, it is rare that a gray tank itself gets clogged.