Look around for the low point drains. One will have a valve and should be the one for the fresh tank. The other ones will just be a small hose. Those are for the lines inside the trailer and the valves are behind a cabinet or drawers inside the trailer.
An alternative is to run a hose from the outside shower to a suitable place and turn on the water pump.
If you are hooked up to a sewer then just open the gray tank valve(s), turn on the water pump and faucets.
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