OP has not been back so we don't even know what tankless they have, all else is conjecture. I doubt it is diesel fired given the coach is a gasser. I have aPrecision Temp 500 and have struggled with it for 3 years -love/hate. When the wind is blowing on the heater it blows out the flame, cold shower follows :( We have added factory suggested fixes and it is better, not perfect. Maybe the 550 would not be as subject to wind.
Freezing temps are another problem as the freeze protection is inadequate when traveling in freezing weather and the hot water side must be winterized before going on the road. Since we travel often and sometimes run into freeze this could be a real issue.
This is still a work in process.