In our two story house the instant water heater is on the bottom floor on North side and the kitchen sink is on the second floor on the South and opposite side of the house. ie about as far apart as possible in the house. We used to waste a lot of water waiting for the hot water to get up to temperature. Installed a D'Mand recirculation pump to eliminate the wasting of water. A small pump is installed under the sink. You press a button to start pump when you want hot water. The water in the "hot" water pipe between the instant hot water heater and the faucet is pumped into the cold water pipe under the sink. A temperature sensor stops pump when hot water arrives at the faucet. When pump stops you now have hot water at the fixture without wasting water. We also have a wireless pushbutton near our shower downstairs. Press that button and a signal starts the pump under the sink. Again you wait a bit and hot water is available before turning on the faucet.I have a high capacity LP Gas tankless water heater for my house. I do not have the RV version, so my remarks are based only on the home system.
Tankless water heaters are not designed to save water; indeed, they waste considerable water waiting for the "instant" heater to heat the water to sufficient temperature and provide it through the household pipes to the faucet.
D'Mand system had better reviews than the similar chilipepper system.
In an RV with limited water supply when dry camping, wasting water is more critical. In the Sprinter I modified a 5 gallon beer keg so I could heat water electrically. All the water in the tank was at the correct temperature so no water was wasted.
The idea works well if you have a inverter powered by the vehicle 12 volt system to provide the 120 volt power to heat the water while driving. Took about 45 minutes driving to get 5 gallons of water up to 90 degrees. A submersible pump then delivers water to shower.
Will copy the system in the Transit build with a few improvements. Change to a 1000 watt vehicle inverter from the Sprinter 600 watt and change the heating element from 450 watts to 675 watts to reduce water heating time down to 1/2 hour. Also gas engine can be idled so do not have to drive to heat water. Instead of a beer keg I will use the wasted space under the portapotti for the shower water tank. The tank will be the support for the toilet.