We are all creatures of habit - I have called it a HOT WATER HEATER all my life and probably won't change now...
The three winterize valves looks like this. Some trailers only have two and still some only have one but the idea is to BYPASS the cold water from going thru the Hot Water heater. You should not allow the RV PINK stuff to get inside the hot water heater. I drain my hot water heater with a port that is on on the bottom side of the heater. I too will leave the relief valve open. There is enough expansion room inside the heater to NOT do any damage from freezing a small amount of water that did not drain out. NOTE the values are not round and will tell you which is closed or opened. If it is inline with the hose it is OPEN.
This is the bottom plug I pull to drain my hot water tank accessible from the outside