Welcome to the forums!
Yep, there is a bunch of plumbing underneath your wood floor, and it connects to the dump valve that is attached to your black tank. To get access to it, you either need to drill out the rivets that hold your bellypan in place, and remove enough of it to access that plumbing, or just cut an "access" hatch into the bellypan so that you can see what is going on. Even with a composting toilet, you will still need a dump valve for your gray water system, so cut a big enough hole so you can really access that dump valve and the connecting plumbing.
Don't cut into the floor if you don't have to (and the only reason you would have to is if your floor is rotting out). The floor is sandwiched between the shell and the frame, and is way more hassle to patch than replacing a few rivets in the belly.