The trailer outlet is a standard 1-1/2" NPT connection. The big box home improvement stores will have threaded fittings. I've decided that the garden hose adapter is somewhat worthless since it is unvented and restricts the flow to nearly nothing.
I use a black ABS plastic fittings to a assemble a very functional system. I start with a female treaded fitting on the trailer to a wye fitting. From the lower side of the wye, I have a 45-degree elbow to a 1-1/2" pipe, 24" long. This goes into my gray water collection bucket (or a 1-1/2" to 3" adapter and stinky-slinky for full hookup sites). From the top side of the wye I have another 24" long pipe going vertical to vent. The vent makes a major difference in the drain operation. Since it is an unpressurized gravity drain, the entire set up is just slid together so I can easily assemble/disassemble.
2004 Toyota Tundra SR5 (V8, 4WD, TP, TRD)
2005 Fleetwod Allegance with axle flip
Honeywell 2000i Generator
Me, DW, DS, DD, & Chicken-Dog