If you're really curious, use what I call the "industrial proctoscope." Mine comes from Harbor Freight
[LINK], but since then there are dozens of cheaper models available that attach to smartphones that you can find on Amazon. It's worth having one of these for general RV work, like scoping out the locations of things above your ceiling (through the wiring hole under the closest puck-light), finding where your clearance light wire fell to after Mickey chewed it in half, and a bunch of other things. I'm suggesting you could just suspend it down the toilet valve and take a look at all four sides of the tank, but I guess you could also snake it under the coroplast and have a looksee at what's there, too.