Wagonmaster ...they will be on the very TOP of a pipe that goes up and stops, likely behind the trap a bit, and definitely ending several inches higher. I find mine more by feel than by sight. The vacuum breakers (they are NOT vents) normally screw on, and will likely look and feel like a cap until you get them off and look inside. Their purpose is to allow air into the tank when you are dumping ...without them, the suction of dumping the tank could suck the liquid out of the traps which would then allow tank odors into the passenger area.
Paul (KE5LXU) ...was fulltimin', now parttimin'
'03 Winnebago UA 40e / '05 Honda Odyssey toad