For an RV, an EZ tankless smallest Direct-Vent model that runs on propane would be fine for mild climates where you want short/lukewarm shower and hand dishwashing warm water. The water plumbing (hot and cold) would be inside insulated space and would not freeze easily if there were people living inside (cabin heater running, even on lowest hat & sweater setting), and it would be safe because combustion air and exhaust only move through the vent, never in the cabin. For cold weather RV hot water, you might want to get a model that puts out 75K+ BTU's (they spec 3.6gal/min @45 degrees temp rise on one). Distance that the hot water goes through pipe and how cold the environment the pipe goes through will determine how much cooling happens on the way to the shower head. Routing and insulation matter! I could imagine a big RV with a clothes washer (hot), dish washer (may have some internal water heating ability), hand washing in sink (intermittent), and a shower (wants steady flow and correct temp), all at once as a worst-case scenario for a heater to handle when unlimited shore-water is available. I would be looking to run one-use-at-a-time as an upgrade from heating bath water on the stove.
If a person stayed in campsites with good electric hookups (50A often, 30A minimum), an electric hot water heater might be a decent investment. I wonder if electric and propane demand hot water heaters could be placed in series to use whichever was available (or cheapest per BTU), or both for "actually hot" water.
Compared to having a 5-10 gallon hot water that a trailer or MH came with, it seems like a tankless system could deliver at least the same quality/quantity of service with reduced weight/space and perhaps save as much as 40% over folks who leave the tanked water heater on all the time (not cheapskates who fire the 6 gallon heater up until it's just hot enough, shower/wash, and shut it down right away until next time to save propane. These folks might just about break even on fuel with a tankless heater, but gain on weight/space/convenience).
Sea-going sail boats often have on-board battery-charging generators that use exhaust heat to heat up a tank of water for bathing and galley washing. A 300cc Diesel that uses a pint of fuel an hour makes >2000W/Hours (DC, often 24v) and at the same time quite a bit of hot water. If you want even more hot water or you have charging capacity unused, an inverter can run hot water heating coils (giving the genset a proper load to work on). Of course, a boat like this has 600+ pounds of house batteries to charge up. Showers are scheduled and short (5 gallons, not a drop more), fresh hot water being precious. Serious boondocking
RV campers should be looking at how folks with sailboats design and build their ocean-going boats where failure is risking death, not inconvenience and calling AAA.
Like most people, we will keep using what we have until it wears out/breaks un-repairably, THEN we will (or ought to) consider alternatives of things like tankless waterheater vs. replacing the old one with a new same-model, or heating water on the stove.