On my late 2009, there are not extra holes for the base. You would need to drill new ones and from what I remember there isnt much room under there for much adjustment until you hit a floor support, but maybe just enough if you're determined.
As for the valve, it just twists on to the outlet in place of the cover, you move the cover to the end of the new valve. I put a little sealent on it to make sure it wont fall off.
This is the one: http://www.campingworld.com/shopping...on-valve/27868
When your gray tank shows full, and your black tank is still almost empty mocking you, leave this main outlet valve closed and open your gray valve, then open the black valve, and listen to the gray transfer to the black. Gravity will try to equalize the levels in the tank. I wait a few minutes then close both.
The extra water in the black tank really helps it empty when dumping. Plus you can use a full gray tank to clean an empty black tank when dumping. Part of my procedure for emptying the tanks is to empty the black first, then transfer gray over to it, close the gray and empty the black again. Rinse and repeat a few times and the black tank has been scrubbed out by the soapy gray water.