The usual water conservation techniques always apply: Take the "Navy shower," as an example. Step in the shower, wet yourself down, turn off the water. Soap up. Turn on water, rinse off. It's practically a contest to see who can use the least amount of water and still come out clean. Other water conservation techniques come into play – save the dish water for flushing the toilet. We've even heard of extreme measures – one RVer uses a bucket and takes gray water out of his holding tank to flush the pot. Not so sure on that one – our gray water never smelled good enough to bring back inside the rig.
But what about wasted water when waiting for hot water to make its seemingly sluggish way from the water heater all the way out to the faucet when you're waiting to wash your hands, dishes, etc? Some (ourselves included) have used a jug or pan to collect that otherwise clean, fresh water, and use it later for "cold water" purposes. But it can be a bit of a hassle, and if you're using a tankless "instant hot" heater, or if your conventional water heater is a long way from the faucet, you can collect quite a bit of tepid water while waiting. Here's an RV modification to think about.
Rather than collecting that tepid water in a pan, instead, simply re-route it with a modified plumbing scheme to send it back to the fresh water tank. How so? By putting a "T" in the water line that supplies the "hot" side of your faucet. One of the T outputs routes to the faucet, and the other side to an add-on line that routes back to the fresh water tank inlet. The trick is to put a valve in the line headed back to the fresh tank. When you want hot water at the faucet, leave the faucet turned off, but open the re-route valve long enough to ensure you really have hot water on location. Now close the valve, and use the faucet – and have hot water readily available.
one solenoid you could use, for less than $25.00.
Of course, you'll need access to 12 volts, and a way to route the "new" line back to the fresh water tank on your rig. It'd really only make sense if your tank is on the same side of the rig as the faucet(s) you want to modify, but hey, it's a great afternoon "mod" project for the boondocker.