Our 2001 2619 trailer has 4 drain valves. Three look alike and drain various parts of the piping system. The fourth looks different and drains the water storage tank. Two are located at the rear of the street side of the camper and two are nearer the front on the same side. If you open a hot and cold faucet inside and then open all 4 valves, your system will be effectively drained of water.
There is more info on the plumbing system in the technical library of this site.
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