I do not think a built in grey water tank on the tug is a good idea, on a hot day it may brew a bit, you could loose friends. I use a 40 liter trundle tank, which I have modified so it fits onto the back of the ute, and now has a lifting handle at each end, so the two of us can manage it if required. It can be wheeled to a dump point or where ever, and washed out afterwards. I also have a built in grey water tank under the van, and have an outlet on both sides, so when I stop for a tea break, I can unload direct to Mr Farmers paddock, and not worry about trundling the grey water anywhere.