Originally Posted by 4wheel1
Front to back which is the black water.
If you can see any PVC pipes and valves below, the black water should be the larger pipe/valve. If you can't and are 100% sure the tanks are empty, you could run a faucet, open a valve with a bucket at the drain and see if the water comes out.
If you aren't sure the tanks are empty, then I'd do this at a dump station to avoid surprises.
Some rigs have two grey tanks, one will be for the kitchen and the other the shower and vanity. If you only find one pull handle and the kitchen sink and shower are on a common wall then you probably only have one grey tank.