I have a State Select 75 gal hot water heater. Mod # PR675NRRS. The hot water thermostat seems to be set at "A". It has not been changed in the last 3 years (since installed). The water temp from the faucet and shower has gotten gradually cooler over the past week or two. What could be the cause of this? There is plenty of hot water but it just seems to be cooler.

When the stat is set to "A" that is not very hot so the first thing I would try is to turn it to the "B" setting and see if this satisfies you. If it does not it is possible that the "Dip" tube that carries the cold water to the bottom of the heater has deteriorated and the cold water entering the top of the heater is mixing with the hot water making it feel cooler than it should be. This dip tube is replaceable by turning off the cold water and disconnecting it and with you finger stick it into the cold water inlet and take out the plastic tube that you can see. It will be white or blue.

As the ground cools, the cold water that's tempering the hot makes it appear that the hot water is cooler. In reality you run of water earlier in the shower because the cold water is colder.