What's beyond the mojave outpost fence because there isn't much room left on the map beyond that point?

The end of the map. The large "box" that makes up the map in the Pipboy has some pretty significantly cut corners, the game world doesn't stretch to the edge like in Fallout 3. The West is mostly mountain, ect. No use feeling cheated, though, because the game world itself is much more detailed and filled than Fallout 3's.

So there is nothing of interest beyond that fence?

It beats the old invisible wall or message telling you to turn back.