Are you sure you have a "mixing valve"? They are used only on the EXT model heaters, the ones that advertise 16 gallons of hot water but actually have only a 10 gallon tank. Standard 6 and 10 gallon units do not blend hot and cold water by design. The 16 gallon EXT heater was an option on the Providence, so yours may or may not have it.
If it's not an EXT model heater, the problem is likely to be a winterizing (bypass) valve that is slightly open or a stickey check valve at the heater outlet. If the check valve sticks and doesn't open fully, cooler water can sometimes enter the hot lines and the symptom is a short supply of hot water.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL