Hank those are the ones. I get mine from Banggood or Gearbest, but the same thing. In reality, I have multiples of them, so I can use AA batteries in anything that needs D cells. Anyway, banggood also sells the 12volt to 3.3volt converter board for like $4. It is real small and fits inside one of the cells. That allows you to bypass the need for D cell batteries and use your onboard system (but you can also use a D cell if wanted).
https://www.banggood.com/3Pcs-Mini-DC-DC-9V12V19V-To-3_3V-3A-Buck-Car-Charging-Step-Down-Power-Supply-Module-p-1123528.html?rmmds=search This isnt the exact one I have but it should work fine, if you want to go the converter route.
As I wrote, with our permanent mount one, we hard wired a switch next to the interior sink, which allowed us to turn the gas on or off (actually it disables the igniter circuit, which also shuts off the gas flow). With the one we use now, we mounted a switch to the front panel, between the heat and water control knobs. Doing this allows you to use a single supply line to a faucet and have a preset temperature. Switch off = cold water. Switch on = temperature controlled water.
With the normal hose length, it takes about 8-10 seconds from turning on the faucet to getting properly heated water. The heater works with city pressure, good pump pressure and even cheap $20 pump pressure.
dave