I have installed Durock in my kitchen floor, and after I glued and screwed it all down, I noticed that the floor is all on one plane, perfectly flat, except for an area that runs from one of the walls to a spot about three feet from the wall. This area slopes downward. So... Can I use a floor leveling product or should I build it up with thinset, let that dry, and then lay my tiles with more thinset on top of that? Any suggestions?

Sure you can use a leveler, there are many. In a pinch and if the thickness is slight, you can use thin set too, but a product made to fill and level is best. Be sure it's a Portland based product. A self leveling cement may work if the rest of the floor is level, otherwise stay away from SLC. How thick would the new fill be?

Jaz

The joists in question dip down about 1". I was thinking of using rapid set mortar in the morning and then coming back in the evening and using regular versabond thin set mortar. What do you think?

By rapid set mortar do you mean thin set tile mortar? If so, NO. Get a patching cement, which also comes in rapid set.

Jaz