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.