The valve marked 'winterize' probably turned off the water flow to the water heater because it's sort of wasteful to fill a 6-10 gallon tank with RV antifreeze.
In this illustration there are 3 valves, the bypass valve is open, the outlet and inlet valves are closed for winterizing. Many RVs don't have the outlet valve, they depend on a check valve at the WH outlet line to prevent backflow into the tank.
Drain the WH be unscrewing the drain plug behind the outside hatch cover for the WH. Open the pressure safety valve to allow air in so water drains from the tank. If it's a Suburban WH, purchase a new drain plug with a new anode attached for when you re-commission the RV.
You mention a manifold, if it is a bunch of valves that distribute water flow to each sink and item in your RV, then you probably don't want to turn off the hot water supply at the manifold, you'll have to find the winterize valves in the line between the cold supply line and WH like in the photo.
Linear Mode
