Jackie,

I can't speak of the Wildcat as I've never looked in the underbelly area of one of them.  However, let me show you a couple of images from a DRV Suites fifth wheel with the layout of the different tanks.  Perhaps with this as an idea, you can slide back the underbelly doors and kind of get an idea of where to look.

In this picture, one can see two different tanks in the area of those two vertical frame pieces.  In this case, the black tank is the upper one and the lower one is for the grey tank.  Thus, the grey tank is right down at the bottom of the frame.  In this picture, you can see water lines going to the top part of each of those tanks.  By the way, those water lines are for the tank flush system.

In this second picture, it is showing the same area, but from the back part of the trailer's frame.  (The white tank right there in front is the fresh water tank.)  If you look beyond the white tank and to the left of the two tanks within that frame, you can see the large drain line of the black tank.  In essence, with regards to the grey tank, its drain line will be down low within the frame, possibly linking up with the plumbing from the black tank somewhere.  At any rate, this will give you something of an idea of where the lines should be in the underbelly, but absolutely no idea of what to look for when looking in that area.  The drain line for the grey tank may very well be covered by insulation.

Terry