Duplicated water heater post here too...
We ended up installing a EccoTemp on demand heater and 20 gallon fresh water tank last week to run the shower and sink. Shower setup is mostly done and works great so far. Sink to be installed sometime this week.
Used a West Marine water pump to supply water to the Eco unit. As soon as the Eco unit detects water pressure it ignites and within a few seconds you have hot water. On the lowest heat and water flow setting it works perfect. About .75 gpm is what I have it set at. Takes about 2-2.5 gallons per shower being quick about it. Yesterday the water tank temp was 62 and water output was 104 and the lowest setting.
The heater is installed above the tank up against the back wall. Photo coming next. The exhaust is minimal at the setting I am using and with all the windows open you can barely tell it's on. May still exhaust to the outside as should be done but may hold off for now.
Testing the unit and fittings before installation.