We dragged our caravan manufacturer kicking and screaming into fitting a grey water tank when our van was built in 2007,,, one of the best moves we made.

We fitted ours across the rear under the back of the chassis,, can easily bolt in OR just make a simple sub frame from steel and Ubolt to chassis. With the tank at rear as we use the water from the FRONT of the axles we deposit it to the REAR of the axles ie better balance.

It's only 60 litres (60 kg max) so realistically in van with ATM of 3000 kg it doesn't upset the dynamic stability of the van.

We can stay in many more places now being self contained IF required.

So what do you need,,,, as you say,, S trap (u bend in the pipe will suffice as long it traps the water to seal it),,, we use 25 mm ball valve outlet (you can buy a 25mm waste hose anywhere),,,, gravity does the rest if it's under the chassis. Suggest you use 20 or 25 mm hose from sink, shower and hand basin to the tank to avoid blockages (ie hair from shower or food bits) and the black convoluted poly is easy to bend - same stuff as you see in bunnings or caravan shops.

So if you need to "hold" the grey water the outlet ball valve is closed,, to empty connect hose and drain either in CP or deep in bush as most do anyway with grey water.

OH forgot 1 thing make sure you have a breather pipe on top of tank ie inverted u tube of say 12 mm tube OR the smell will regurgitate back up the line,,,lol.

Good luck

Thanks Peter & Baz,

Looks like it is a goer.

As soon as the rain stops for a bit I'll look under the van for a place to fit another tank.

If one can go near the current waste pipes outlets, I might just put the holding tank inlet near them & use short lengths of hose via an S bend & a solids filter trap to connect the outlet to the storage tank.  Only issue then is conveniently locating the empting outlet with a ball valve as suggested.

I'll see what bits my plumber son can supply.

Thanks again  ..  G