Had problems recently with the temperature of the hot water coming from the tank. It had always been VERY hot and we never ran out. On the last trip we discovered the temperature was much cooler and you could hold you hand under the kitchen sink faucet for a while without too much pain. I figured the thermostat had gone bad so when we returned from the trip I installed a new one. However this did not correct the problem!
After posting my problem on another forum I quickly found the answer to my problem. When the outside shower is left with both handles turned on and the small shut off on the hand wand is turned off you are not aware of anything. Water does not pour out of the access door and you would think everything is ok. However the hot water gets a feedback from the cold side of the outside shower and this cooler temperature goes throughout the plumbing system!! I could not believe it when I discovered mine fully on (probably from winterizing last Fall) so I fired up the water tank and soon had 140 degree water coming from the sink faucet. I went back outside to the shower door and opened both valves but left the hand wand valve closed. Now I had 110 degree water coming from the sink faucet again!!! So beware of this problem, I think many people fall into this trap.