Grey waste tank
I added a grey waste tank which I had custom made at the same time that the fresh water tank was made. This tank holds 85l. This again is probably far too much and a 50l tank would have been ideal. But the good thing is you don't have to fill it up! Most caravan sites have drive over grey water dumps so that you can empty them before you leave. This tank is only filled with shower and sink water so it doesn't really ever get too full anyway. I added a tank level sensor too.
Before i mounted the tank i coated it with two layers, first one is insulation to stop the water inside freezing when i take the van to europe skiing and the second is a black plastic cover just to stop stone chips from flying up off the road and scratching it. It also helps trap another layer of air between the two for more insulation.
I mounted mine under the back of the van in between the rear axel and the spare wheel. There is a small space which is known by the crafter community for holding these tanks. I made the tank fit this space perfectly using every inch of space - again in hindsight this admittedly was over kill. I then fashioned some hooks out of long metal screw thread canes and made a support cradle out of some left over metal from the seat project. I coated them all with several layers of paint and then a final coat of waxoyl to protect it from stone chips flying up.
This took no time at all to make up and then finally the last bit was added the release valve - this is a solenoid connector (12v) which i have rigged up to the control box at the front of the drivers cab so that you can empty the waste from the drivers side...this is obviously only for dumping in designated areas but it makes it easy to do so. The end of the release valve is a pipe which goes to a connector I made on a lathe which has a screw thread so that i can connect another pipe incase i need to reach across to wherever the dumping area is.