Someone mention my name??????
So far, there are 3 RVIA approved tankless water heaters available certified for RV use: Precision Temp's RV-500, The Girard Products unit and the new Atwood Tankless unit.
The RV-500 is a solid unit that has been pretty well discussed and tested. I had a Girard when it first came out and didn't think it was the effort to connect and test it.....just didn't look like a quality unit. The new Atwood unit is just a less in price than the RV-500 but comes from a company with many years in the water heater, furnace and RV products business and looks like a well built and substantial unit.
Let me know if you have any specific questions.