Water heater mystery

[Runaground]

We have a 2016 AS International FB with an Atwood XT water heater electric/propane fueled. After properly filling with water the heater fires up perfectly on my choice of electric and/or propane. When showering or running any faucets we get only 2 gallons if suitable hot water before it turns to ambient temperature water. When examined the water heater still contains extremely hot water --just none to the taps. The winterization ball valve is in the "in use" position. The shower faucet blender and controls are off. The are no other in line valves on our unit other than the system drain valve and of course the mixing valve. What's up?
If, as I suspect, the mixing valve is corroded I'm having no luck getting to the adjustment feature behind the black plastic cap and I can't manage to get the valve loose from the tank even though the hot and cold connections can easily be removed. If this is my problem, how can I flush this valve in place? I've read the vinegar flushing articles and am willing to try that to clean the tank (I've already flushed it 4 times with plain water) but how do I get the solution through the valve?
Any suggestions as to what's wrong?

[Runaground]

I'm answering my own mystery here: it was the mixing valve! A small stem was not seated properly and punctured a screen on the brass connectiobhn between the water heater and the actual mixing body (it's still a part of the mixing assembly)itself. One may be able to order just that brass inlet fitting from Atwood but I went ahead and ordered the whole mixing valve from Amazon Prime for $116. Hope this helps any other sufferers from cool showers.