Viewing pics of that floorplan , just my opinion:
I would think its a setup as you showed with a cover on it.
If the valve was located under the bed, I'm assuming the tank is also located under the bed due to that floorplan and the location of the outside gravity fill port.
Its a driver side full wall slide floorplan which would leave the passenger side to house the water tank which usually needs a solid floor for support opposed to a slide floor. It appears under the queen bed is the only place for the installation of a water tank in the 50 gallon size range.