This (busted) valve is set so that the bottom of each picture is the fitting that would have gone to the waste-line. The top (black gasket) goes into the black-plastic tank fitting that attaches to the tank (gray or black water)
The two pictures show the gasket bevel up and bevel down. Its not as obvious as it seems because the tank fitting that fits onto it is also flat with a ridge around it. So, which direction should the gasket be set?
I've got it set in the first one - bevel up, flat side on the valve. You can see a closeup by clicking on the picture
Hope I'm right because I'm really getting tired of dropping the belly pan and opening up the black tank!
(for the record, this is Thetford model 08709)