Buy a Sharkbite check valve.
LinkCut the red PEX tube coming out of the water heater and add push the valve onto the line being careful to observe the water flow arrow.
Are you having a problem with the cold water blast in the shower or does the one way valve just seem like a good idea.
Many RV water heaters already have a check valve at the exit line, it's possible yours already has this.
Does your RV shower head should continue to dribble water out of it even when you flip the switch off. This is to prevent the cold water blast as the pressure can't build up enough to backflow the line.
If your shower head shuts completely off, you can avoid the cold blast by only shutting it mostly off. Leave a little dribble out of the head and the cold blast won't occur.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup