The valve is about 2' from the side of the trailer, like Jim said its located in the center of the rear quater panel. If you kneel down you should see it. It will be a slow drain, if the tank is full plan on spending some time. I also open the hot water valve to drain as much as possible, and removing plug at the bottom of the water tank seems to help as well. I've never tested this, but seems that turning off the pump and leaving the hot and cold water valves helps a bit.
Yours has a petcock? The only way I have found to drain the fresh water tank on my 26RS is the low point drain, which is just a cap that unscrews. If there is a petcock somewhere under there, I've missed it.
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