From what I understand there isn't any way to adjust the temperature in your hot water heater. What brand is it?
From what I have found, a common reason for this is the bypass valve(s) are partly open allowing cold water to mix with the hot. The only other thing might be the high limit cut-off is bad, cutting off the burner at too low of a temp.
I'd check the bypass valves first.
Do you have a drain plug or a drain valve? If it were me, and it had a drain valve, I'd get the water heater up to temp, turn it off, and with no pressure on the system (pump/city water off) crack a hot water faucet momentarily to relieve pressure, then carefully open the tank drain valve enough to see how hot the water is. This would tell me if the water in the tank was actually up to temp, because if it is, and your bypass valves are closed, one could be leaking cold water past a valve.
If you only have a drain plug, you could do the same by carefully lifting the pressure relief valve but be aware that sometimes older PRFs won't fully re-seat and can drip afterword. When they do sometimes opening and closing them a few times works, and sometimes it doesn't at which point the valve should be replaced. You may need pump pressure on to get anything off the pressure relief valve since they sit near the top of the tank.
Please be careful either way and assume the water coming out will be very hot.
If you find the water isn't hot after the burner shuts off, I'd suspect the high limit switch. They are usually two round side by side devices with a pair of wires going to each round switch and can be located behind a cover somewhere near the control board. Google Atwood or Suburban hot water heater hi limit switches and you'll see what they look like. They aren't expensive.
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Tom and Pris M. along with Buddy the 18 year old Siamese cat
1998 Safari Serengeti 3706, 300HP Cat 3126 Allison 3060, 900 watts of Solar.
Dragging four telescopes around the US in search of dark skies.
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