Most rv manufactures today install water heater by pass systems on the rvs they sell.
Rv hot water heater bypass two valve diagram.
When the by pass system is used it takes the cold water going.
The next case is one is which you have a valve on the cold water line and a bypass valve in the middle but there is no valve on the hot water line.
You may need to remove a cover to access it.
Cold water comes in at the bottom and hot water comes out of the top of the heater.
This is called a back check valve.
The by pass system can be used to drain the water heater and to winterize the rv water system.
What you will see instead of a valve is a brass fitting at the hot water outlet on the water heater.