Rather than "snake" it back and forth, make it like a house radiator:

||================||
||================|| Sink
||================||
||================||

( artistic note the || & || are all connected)  It must drain to the rear (one
would want the water to settle in the low point), because if it drains to the
front, you will get airlock and the sink will not drain properly (maybe not at
all).    malcolm s

W. wrote:

>  The only thing I can't figure out is how to keep the water from setteling in
> the low point - that is from where you come off the gray water tank, go lower
>  and travel across the van back up to the sink drain fitting on the other
> side.  Water seeks its own level so you would always have water in the line
> between the sink and the gray water tank-you wold also need a "trap" of some
> sort ( with a clean-out)  or you would constantly have that "grey water
> smell" inside you van.