I don't know that trailer model, so this is just guessing, but there may be more than one low point created by the plumbing lines changing elevation along the length of the lines. In fact, most trailers probably don't fully drain from a single low point location.
As for no valves, mine doesn't have valves either. Just blank end compression fittings, which are the equivalent of a pipe camp. Easy to install valves if you want, for future convenience, but they chose to save a few pennies and not include them. Fine by me, since we don't winterize here in del norte.