Bingo - in all seriousness, here's what I know on your 3400,
The tanks span the width of the trailer and run frame to frame. The forward tank is the gray water, next is black, then galley, and freshwater in the rear.
As you can see in the photo, (right being front) the sink and shower drain forward and drop into the gray tank. There is a vent stack through the roof behind the sink for the gray tank. There is also a vent valve for the tub drain located behind the center access plate on your tub.
The toilet drains straight down to the black tank. There is a vent stack through the roof in the wall between the bathroom and hall.
The W/D drains direct to the main sewer line. There is a vent stack through the roof in the wall between the W/D closet and the hall.
The kitchen sink drains into the galley tank in the space at the end of the kitchen counter. There is a vent stack through the roof in the wall behind that little cupboard above your counter. There is also a vent valve located behind the access panel at the end of your counter.
The vent valves allow air into the drain line to help displace the water as you drain the tanks. They just screw on to the end of the pipe.
Hope that helps