The waste water comes from three sources, kitchen sink, bathroom sink and shower tray. The toilet is a self contained cartridge so no messy plumbing there.
All waste water accumulates to the waste water tank under the van, at the lowest point but that hasn't worked out perfectly.
For the kitchen sink and shower tray, standard household 40mm plastic components have been used and this give fast running, clog free waste water flow. Because the bathroom sink ended up being a micro-caravan style affair, we had to fit one of the small bore drain pipes and the difference is obvious, almost feel like trying to push the water down the hole, but it does go.
All the drain pipes connect together and run out through a hole in the van floor and into the waste tank via a length of flexible 40mm hose. This connection was chosen to provide a bit of 'give' between the tank and plumbing when the van was driving down the road and so prevent damage due to vibration. Not sure if this was necessary, but it felt better.
To empty the waste tank, there's a large lever operated valve connected to the tank drain hole and then to a 1m length of hose. Finding somewhere to hold this up underneath the van was a bit of a problem but eventually it came down to fixing large clips intended for holding tools to the chassis of the van. Getting the heights right so that it was easy to access to pull out and put back was achieved with wooden blocks.