Stop wasting tank water waiting for the hot tap (Enviro Save Water System)

Every time you crack the hot tap on a caravan running an instant/continuous-flow gas hot water system, you tip out a slug of cold water that's been sitting in the lines before the hot finally arrives. On the road, that's not just wasted heating — it's precious fresh tank water going straight down into the grey tank.
The Enviro Save Water System (enviro.net.au, AU$220 incl. GST) is a neat fix. It's a thermal valve plumbed into the hot line. When you turn on a hot tap, the initial cooled water passes through the valve and gets diverted back into the cold water line / fresh tank instead of running to waste. The moment the water reaching the valve hits the correct temperature, the valve automatically switches over and sends hot water straight to the tap — so the very first drop out of the spout is already hot, and the cold slug has been saved rather than dumped.
It comes in two configurations: System A recirculates the cold water within the system (via a pressure tank), and System B sends it to a separate tank or container. Both work with either two-tap or flick-mixer setups. It famously won the judges' and people's choice on the ABC's New Inventors.
For a van, the win is obvious: when you're free-camping and every litre in the fresh tank counts, you stop pouring 1–2 L down the drain every single time you wash up or shower. Worth pricing up against a plumber's hour at the next service — it's a one-off install with no ongoing power cost.
Caveat: it's designed and marketed for houses, so confirm with whoever does the install that it'll suit your van's plumbing layout, pump pressure, and the continuous-flow heater before committing.