The valve is usually "AT THE TANK CONNECTION" (not downstream from it on a 3" pipe). If it were "downstream" then heavier solids would migrate to the pipe, clog it and you'd have a "big mess"... So, the valve is "next to the tank opening" and the handle is either attached by a solid rod or a "Bowden cable" (similar to the engine control cable on your lawn mower). Typically, if there is a solid rod, it exits through a slit in the bottom cover of the trailer. If there is a Bowden cable, it is either in a convenience center or secured in a 3/8" hole drilled through the frame rail.
There is a valve, it needs to remain closed except during actual dumping and unless someone "took it off" or "stole it", you'll find it, probably under the trailer at a slit in the bottom cover or tucked way up high under the skirting, on the frame rail. It's probably a 'dull black T handle" so it won't be easy to locate in the shadows, but it's there...
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS