40gal.rheem gas water heater 9yrs. Old (no expansion tank) water heats properly but, can only receive around 20 gal. before it goes luke warm.I have even tried turning up the therm. For hotter water to try and blend it with more cold to get desired temp. needed.NO LUCK. Could I try replacing the plastic mixer tube 1st. if so what are the steps required for this?
Sincerely, mr. frustrated

Turn the water off. Disconnect the cold water line going into the water heater. When you get all of the pipes out of the water heater you should be able to look down into the hole and maybe see a piece of plastic tube, this could be white it could be blue, and it is possible that it could be gone all together. If there is something there pull it out. If it is good it should be about a foot shorter than it would take to reach the bottom of the heater. But if it looks like it has deteriorate then replace it with a new one. Put every thing back together and turn the water on.