"The one thing in common is that all waste pipes will take the shortest practical route from sink or shower etc to either one side exit or the other, or to the rear of the van"
Sorry Damian but I disagree.
Most vans now seem to be intent in getting all waste outlets to the same point which with my van (which has a centre kitchen and end washroom) means quitelong runs under the van.
I found that this means there is a small loop where the corrugated pipe meets the rigid pipe and thus the out flow of waste water is restricted which in turn means a) it can freeze in winter and b) food scraps can get trapped and cause odours and c) the outflow is pitifully slow.
In my case i detached the corrugated pipe from the rigid pipe at both the washroom and sink ends and the put the corrugated pipe straight into the waste water containers. Yes this means you need 2 but the water no flies out when you empty the washing up bowl, or have a wash or shower and it never freezes or has unpleasant smell.
If you come to sell the 'van just reconnect the pipes.
I've fixed a couple of "C" clips under the 'van to clip the corrugated pipes to when travelling.
Hope this helps
)