My 6 gallon three way tank (heats from engine, shore power, or propane) works well with a recirculation loop. The 1 amp wet rotor pump is quiet. I have 2 second hot water everywhere in my Barth. The loop holds about 3/4 of a gallon. There are back flow preventers to prevent cold from pulling from the opposite side.
Check the rise at your gallons per minute on the tankless you are considering. You may not get the warm water you are hoping for.