yep, that's your WH bypass valve for winterizing. the WH's have a preset temperature. i guess you could use the bypass valve to vary it. i've never found any RV that had water that was too hot and needed to do something like that.
I think I figured this one out. There is a bypass between the hot and cold lines on the output side. It was turned on so I turned it off. I guess you could "feather" it to get the desired temp.
Tried it on gas and also electric. Burned out electric element (from being on with no water in the tank) is very common and a in-stock part at every RV dealer because of it.
Also see if you're getting warm water of the cold tap too. If the shower is turned off a the head it acts like a mixing valve. The outside shower is infamous for this and is usually locked in the lil' compartment.