I recently communicated directly with Kyle Johnston at Girard and asked him about the new heater. He explained that the new version incorporates flow and temperature sensors so it can better regulate the outlet water temperature. Doing so will also allow you to add cold water into the flow at the fixture, something not generally recommended with the current heater version(s).
Specifically I asked him whether the new unit would better serve those of us who boondock regularly and like to take "Navy" showers. The news was not all that encouraging. He stated that their tankless heaters (both new and old versions) are designed to function best with a constant flow of water start to finish, not cycling the flow on and off or reducing the flow to a mere trickle. Too low of a flow and it cannot reduce the flame enough to keep the water from being too hot, so instead it shuts off the burner and you get cold water filling the line until you resume sufficient flow. A regular tank-style heater is better at dealing with these operating consitions since reducing the flow to a trickle just allows a smaller amount of already heated water into the line. Turn it back on and (except for slight cooling in the lines) you immediately have hot water again. So it looks like my permanent solution is to retrofit a tank-style heater into my Seneca. That also will give me the option to electrically heat my water when plugged in at a campground. I miss that option too.
Kyle did offer me a great price to convert to the newer unit (new heater only, installation on me) telling me that the dimensions are very slightly larger, but he believes most cutouts have enough "wiggle room" that only slight massaging may be necessary. But he also said the water inlet and outlet are on the opposite side as compared to the older units, so plumbing will also have to be modified. And the door and attachment is completely different, so if you have a painted door (like I do) you will also have to factor in a paint job for the door if you want it to look "factory".
I had so hoped Kyle would tell me this would be the panacea for boondocking, but alas, it is not. But he was very honest and helpful, so I have to give him credit for that.
Anyone have any experience with a 10-gallon Atwood XT?