You're asking about drains for the fresh water system, right? Typically, there are 3 of them. One drains the fresh water tank, and then two low point drains to drain the hot and cold water lines. Try looking under the trailer for water lines that stick out the bottom. There should be a drain valve for each one inside the trailer.
If you have a roof a/c, there will be an extra drain line directly in front of or directly behind one of the wheel wells for it. This is typically a vinyl tube, and it'll look different than your water line drains, which may still be be copper, unless a PO replaced them. You also may find a drain for your fridge, but again, that will be a vinyl tube that will look different from the water drains.