I had a 78 and there was a drain valve under the floor for the main water tank. On mine, if you followed the pump intake hose and water fill/vent hoses there was a section cut out of the flooring to get them to the tank. Way down in the bottom of that cutout, if you can see it, and get your hand there, you may find the drain valve.
Now on mine the original valve didn't work well at all to completely drain the water out. I found the pump did a better job. What remains is not enough to cause any problem with freezing in the winter.
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