I never thought of Aqua Hot (or Newmar's Oasis) as being tankless, though I expect that's true, or nearly so. But they're not something that's easy to add on to a coach, they're expensive, and they tend to be a (relatively) high end option on diesel coaches - I'd not want to use gasoline in lieu of the diesel fuel.
That being said, they are nice, indeed, and will give as long a hot shower as you have water for. They're generally tied in to engine heat, also, so when you arrive you already have hot water. The AH is also the heat source for your furnace, on coaches so equipped, so it's a bit more than just a water heater.
So far, the only time (with either AH or Oasis) we've had to run it on diesel (when hooked to shore power) is in very cold weather, when electric didn't keep the furnace hot enough.