This drain should only be for the fresh water tank. The plumbing line low-point drains (Hot / Cold) should poke through underbarrier as (2) 1/2" (approximate) diameter pipes...My drain valves for these pipes are inside the camper directly above where they drain out the floor. (see picture above)
On my TT - that exact valve swings 180 degrees open to closed and only drains the fresh water tank (from a tube out the bottom of the fresh water tank to that spigot). If your fresh water tank was filled, gravity pressure should cause it to drain. I cant imagine you have "vapor lock", but you might want to loosen the inlet cap and make sure the overflow vent is not blocked when opening fresh water tank low-point valve.
Usually low-point water line drains are installed just in front of the pump. I usually depressurize and open up all my spigots before popping the low-point water line drains.