I removed the original (I think) grey water tank from our Scamp
. Then I went with something similar to what Pete described above.
I checked at a local restaurant and was offered two yellow rectangular mustard bottles? that have a capacity of about 3 gallons each. With a drain similar to what Pete described I can move the drain hose to either bottle and normally dump them when each are about 1/2 - 3/4 full.
One trick I read on the Airhead toilet site is to drop a sugar cube in your "yellow water" container before use. That eliminates any odor build-up. I find it also works for the grey water.
Incidentally I am strongly considering installing an Airhead in our Scamp. Wonder if anyone else has done it?
(Air Head is a composting toilet that is specially made for boating/yachting applications. They also have a competitor called Natures Head.)