Underneath our caravan sink was a mess of plumbing. As you can see a large fitting and poorly routed hose stole most of our already limited storage space. I purchased the much shorter and more compact fitting online and installed it myself (rerouting the drain hose in the process) and now have space for another storage box.
A comparison of the two fittings. The white one is nearly 120 mm tall and the one on the right is 35 mm.
Lots more space now:
The plumbing which remains still leaves much to be desired but it would be lots of work now for little gain (save neatness) but when the caravan was being manufactured it would have been (relatively) easy to install the plumbing in a workmanlike and tidy fashion and to have the caravan delivered with storage containers which actually fitted the space. Every caravan whose cupboards I have opened seems to have just the same issues – yet many folk paid tens of thousands of dollars (more) for their vans than we have.
There are many such low profile right angle sink outlets available but it is often difficult to work out which one would suit (as the dimensions are not always clear. I used this one from http://www.jpwmarine.com.au/products/plumbing-toilets/sinks-waste-plugs/sink-waste-plug-90dg-25mm.aspx which suits a 2″ or 5omm dink hole. The Smev one available eg from Caravans Plus suits a much smaller hole.