The laundry detergent jug to which I referred in the previous post is quite
similar to a plastic gasoline container.  It has a "loosen to vent"  screw
off cap near the top of the handle and a rather less easy to screw off cap
with a built in spigot.  You can very easily modify the spigot cap to accept
your hose using a dangerously sharp implement and adding a rubber grommet to
effectively "seal" the hose when attached to the wastewater container.  Put
a large meadowmuffin on the dash to remind you to take the container with
you when you move the van.  Or you could use a red gas can which has all the
same features pretty much.  However you have to BUY the gas can BUT in a
pinch you can carry gas in it.  If you empty it often it doesn't get too
groady.