I don't have a motorhome at this time, but we do use a Natural gas tankless water unit at home, a drawback that I can see for an rv is the unit wastes a 1/2 gallon or more of water heating up.
When you turn on the "hot" water valve the unit kicks in an starts heating water, and will as long a water flows thru it, of course the first couple of quarts are cool- then they get hotter as the unit reaches its operating temperature.
example; When showering at home, I'll turn the valve all the way to hot, close the door, put the bathmat off the shower towel rack onto the floor, then reach across our bathroom to retrieve my towel, hang it next to the shower, then check the temperature, its usually just then getting warm enough to enter the shower by that time. Note that our tankless unit is on the wall directly behind the shower. It's great for our utility bill, and I love the fact you can shower as long as you want or two showers can run at the same time, or I can fill the wirlpool tube without running out of hot water which is why we converted to tankless in the first place. I just think in an RV you would want to be hooked to a city supply.
Also in our kitchen we have found that you should always crank all hot first as half and half will not be enough water moving to turn the unit up to get water hot enough for rinsing dishes.
At our office we had a 120V tankless unit installed it was a waste of plubming it would not even heat the water in our bathroom sink. It was 1500 watts, like a hair dryer trying to heat water. we replaced it within a week with a 3 gallon electric unit.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio