You'll have to crawl under the trailer... It isn't very big, IIRC, only a couple of inches long and the same color as the tank. It is of the "petcock" variety and, again if IIRC, kind of "L" shaped and on the back/rearward side of the tank on the bottom edge. Remember the position of the handle and put it back in the same spot; it doesn't have any "detents" or other markers to indicate open or closed.
All the water doesn't have to be out for freeze protection...a thin sheet of water or a couple of puddles (my tank had "saggy" spots) won't hurt anything if they freeze.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)