Thank you for all of the replies!
I will have to look when I get home as far as the make of the water heater, etc.
The water inlet comes in under the sink of the kitchen of the camper. The hot water heater is adjacent to this. The plug I serviced was the drain plug on the outside of the camper.
I thought it might be from the sun as well, but the issue was repeatable in a short period of time and also in the morning.
I believe that the cold water runs into the camper and straight to the kitchen sink, but I'll take some pics tonight.