The issue is variable flow rate and temp rise. Tankless systems operate on GPM flow rate. The more flow (demand) the hotter the burn rate but there is a minimum of GPM (gallons per minute). They also operate on a premise of required temp rise. So combine the 2, there is X burn rate required to achieve Y temp rise at Z flow rate/demand. The reason they work so well in houses and so horribly in Rv's is that these components are static in a house but variable in an RV. Not only is the water temp and flow rate different for every CG, it's also different for a water pump vs water connection. And usually both are less than the requirement for a tank less to function optimally add to that, flow rates - especially on the water pump fluctuate/cycle. Too low a flow rate and they won't ignite at all. Flow rate and temp required for rise are pretty constant in a house and the flow rates are usually adequate as is required by code. There is no code to ensure high enough flow rates in a CG or an RV. If you could ensure consistent flow rate at the consistent minimum GPM and consistent water source temperature, they would work fine. But reality is, an RV provides none of these. Exactly the opposite.
