You don't say what kind of races. MX maybe?
The gray water should not really smell if just dumped out, it will start to smell bad inside a closed container however. I dump mine straight to the ground everywhere. Gray water is not pollution and a thimble full of soap is not bad for anything either. I even dump onto pavement if the water can get out of the way without others around having to deal with it; it does not have any odor. Sink water is mostly water and soap and whatever is on your hands or left on your dinner plate. Clean the cookwear and plates of debris as much as possible as you dont want particles in your sink hose anyway, and this will keep it off the ground.
In some parks dumping gray water will not be allowed so a bucket under the hose is all that is needed and that bucket has a way of getting disposed of each day.
A lot of cargo conversions has just a 5 gallon container to collect gray. This is not rocket science. If the container can hold liquid then it can hold gray.
