Well after a couple of little trips, the sink was annoying me, it would take ages to drain. A little investigation, any wonder! Drain pipes that look like a snake and restrictions in fittings etc etc. I know they do it cheap because it's a caravan so I had a play with it. I only have a sink so I did not have to go to the trouble Brad did in his 18, I do not have a shower. I decided to use 20mm pressure pipe for two reasons. You can get it from Bunnings cheaply and it fits perfectly in between the chassis openings.
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Look at the restriction in the pipe your sullage hose hooks up to and the bent drain pipe. Even the fitting in the sinks is restrictive.
I stripped every thing out and started again. The clear plastic drain pipe under the van was all over the shop, like they have one length and that fits all. The main thing I thought was wrong was no air can get in the head (sink end) of the drain. I fitted a air admittance valve, over the top but I did. Then I just ran the 20 mm poly and fitted a barrell union at the hose end so I dont have to stuff around with my sullage hose. I fitted a home made p trap but you would not have to. I just used standard plumbing principles. Straight pipe and air in drain, just like home.
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I lost a little bit of space, but the drain works great so I can live with it and my wife does not even know I have done it!
Cheers Geoff