doesnt the water heater have a thermostat?
on the girard heater, it has a digital thermostat that lets you set the hot water to whatever temp you want. that way you turn the water all the way to hot, and its stays there.
the flow shouldnt have anything to do with the heater, unless there is a restriction there. thats either the pump of the shower head. construction debris in filters, or lines can be the cause of poor flow.take the shower valve apart, and check for shavings. on a newer coach, thats always a problem.
if you dont have a variable speed pump, water flow will pulse. the
temp will fluctuate. getting it set to your preference should not be that hard. its a real learning curve from the old style heater, but i am happy with mine now. earlier, not so much. it just took too much adjusting to get it to work right.
the digital thermostat made a night and day difference.