Most rv manufactures today install water heater by pass systems on the rvs they sell.
Rv hot water heater bypass two valve diagram.
You may need to remove a cover to access it.
If your rv has a bypass valve these two pipes will be connected directly to each other using a third pipe.
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.
When the by pass system is used it takes the cold water going.
This is called a back check valve.
Cold water comes in at the bottom and hot water comes out of the top of the heater.