On my 78 Excella 31' center bath, the drain valves are in the wardrobe across from the bathroom, near the water pump. See if you see them there. If not, I have a 79 Excella manual at home I can check for you. I would guess they are in the same place as the Sovereign.
I already have studied the area in the closet that you refer to but for the life of me I do not see any manually operated valves.
I have service manual for the unit but it does not point out the location of the valves. I actuall says that you can find the drain location on page 5-14 of the service manual but there is no reference to the drain valves on that page or anywhere that I can find.
The manuals are definitely not clear in describing where the valves are!! There should be a diagram in the manual indicating a general location of the valves though. If you don't have it, I'll get it for you.
Try looking at the unit from underneath. Where the drain valves are, there should be holes in the pan for a tube for the water to escape. The valve would be somewhere above that.
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