Looks like the FW drain on my trailer. Should be a cap on that line though.
Another tip that it's FW drain would be is it located near or between those 2 u-channel's under the trailer that hold the FW tank up? If it is, you confirmed it's the drain. Mine is straight out of the tank with no valve, just a cap that I take off to drain.
Just the 2 of us in a...
2015 Freedom Express 246RKS
2002 GMC Sierra 1500