Should this have been figured out by the rv manufactures before they sold it as "standard equipment"? I'm not one to "jury rig" a water heater to make it produce a steady stream of warm water. The modern tank water heater is so well insulated it takes a whole lot less water and propane to create a steady stream of hot water.
Less water, less propane, seems the old tried and true tank heater is still a much better value than an on demand water heater that needs near perfect conditions to perform. After having a moment of scalding water followed by ice cold water does not in any circumstance make me a happy camper. The on demand FAILED to deliver the hot water needed to wash dishes or shower.
Sorry but for the price paid for a motorhome one should not have to hope for a steady stream of warm water, nor should we have to problem solve a failed system.