The OP did PM me but for anyone else out there......
You have to take the bed apart. You need to remove the matress and the support. You will then see the valve to drain the tank. It is a dumb design. Also a pain to replace the water filter and winterize because you need to take the bed apart.
There is enough PVC pipe sticking out of the floor for the drain under the coach that I attached additional PVC and I installed a drain valve under the coach. I leave the factory valve open and then use the valve that I installed to drain the fresh water tank.
This spring I am going to install a electric valve so I can just flip a switch to drain the fresh water tank.
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