Just spoke to Shaun at http://www.tank.me who fitted my underslung tanks.  He is sending me a straight through connector.  I must recommend Shaun yet again for his exceptional service and great price.

I have just crawled under the van (Yes in the snow) and cut the pipe right next to where it exits the floor of the van.  Unfortunately the only place I could lie to cut it was right underneath it so I got my arms absolutely soaking in freezing cold water.  However I now have 2 working.  The kitchen sink and the shower are now fully operational.  Just need to do the bathroom sink tomorrow.

When I do the rebuild I will be rerouting the interior pipes to exit the van close to the tanks.  I will then put a t piece where it exits and fit a drain valve.  Then in the winter I can turn the valve to open and the waste will come straight out rather than going to the underslung tank.  I unfortunately can’t do this for the shower but the shower disconnects easily enough underneath.