We had precisely this problem.
The fix was to replace the tempering valve that controls the temperature of the domestic hot water. On our coach, that valve is located on the outlet side of the hydro-hot where it can be easily accessed. the valve was about $150.00 from Monaco parts (it's a Honeywell valve, no doubt available from plumbing suppliers) and took about 30 minutes to replace.
Since replacing, the domestic hot water temperature has been stable making for comfortable showers.
2006 Camelot PAQ
RR8S Chassis, Cummins ISL 400 hp
Ford Flex Toad