I have the Atwood tankless, which is pretty new to the market. Maybe only about 1 year old now. They licensed some of the technology from PrecisionTemp...
Precision Temp teams with Atwood to design tankless RV water heaters | RV Daily Report | Breaking RV Industry News and Campground Information
The nice thing about the Atwood is that it fits the rectangular cutout that is common on our airstreams. They are also cheaper.
I have mixed feelings about the Atwood unit. I generally like it, but performance is a bit all over the map. This may just be the nature of the instant units -- as 65CV suggests, they only boost the water temp a certain number of degrees and that number changes based on the flow of the water. The atwood has a winter and summer mode to compensate a bit but on really cold days, you'll definitely notice that the water isn't coming out as hot as other days.
The manufacturers of the instant units try to dissuade you from taking "navy" showers because there are drastic temperature fluctuations when you do... well at least on my Atwood. The heating coil gets very, very hot, so when you stop the water flow for a short period, the water that is stuck in that coil can rise to very high levels (even though the heater turns off, there is enough residual heat to really get it up there).
BUT I CAN take navy showers but just have to deal with the quirks. Mainly, I just wait for a few seconds for the water to stabilize again, as you would with a regular heater, but the difference is the way the temp fluctuates in that period.
The net effect when I restart the water is the following sequence:
(1) first, you get lukewarm water that was in the pipe between the heater and the shower, which has cooled slightly from sitting there. (You'd experience this with a regular hot water heater as well.)
(2) then you get the super hot water that was sitting in the coil, as described above.
(3) then you get gradually warming water again, similar to what you experience when first turning on the unit and waiting for it to warm for a few seconds. This restart is faster than a cold start, however.
My unit likely wastes a tiny bit more water than a regular water heater. In a regular unit, water after item #1 above will be at the correct temp. So the waste in the tankless unit is water required for #2 and #3 above.
So as you can see, it can be a bit quirky, at least with my Atwood unit -- but again, this is ONLY when you are taking "navy" showers. I would think that the PrecisionTemp might have similar quirks but someone else might have to chime in about that who knows.
Would I buy it again? I'm on the fence, but probably. For me, I just have a hard time thinking about the water heater constantly running, or needing to plan to use hot water to give the unit time to heat. Yes, people will probably respond and rationalize how overall the 'old' heaters work better for a lot of reasons, and they have valid points. I do like the "on demand" aspect quite a lot, and I really, really like that when I have hookups I can basically take as long of showers as I wish. (And note that if you have hookups, you don't really need to take "navy" showers anyway and you won't experience any of the quirks described above.)