The fresh water tank drain valve is likely very near the fresh tank and probably on the bottom of the trailer. I don't have much experience with trailers other than mine but it may provide a guide that will help you.
Where is the fresh water fill on your rig? It's probably in a trap door on the side of the trailer. It's also probably near the axles to keep the trailer pitching wildly with a full tank of fresh water. Now look on the bottom of the trailer in/on the belly pan area. I suspect there is a valve of some sort in the area near the fresh water fill.
My fill door is just above the tires. My drain valve is located between the tires at the frame / belly pan.
Lucius and Danielle
1992 29' Excella Classic
1996 GMC Suburban C2500 7.4L
2005 Chevrolet Suburban K2500 8.1L
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...