My 2012 is probably very similar to your 2016. I have an open underbelly. There is no piping in that area. The FW tank is above and between the axles. The Grey water tank to the rear with a bit of a gap, then the black tank, is behind that under the toilet.
The sink drain runs through the inside of the TT, until it drops through the floor and directly into the grey tank.
By chance is the FW tank overflow tube sticking out of the Coroplast? Could it have been snagged and is now discharging into the enclosed underbelly. You can gain access to the over flow pipe from the water pump location. On mine it was only held in place by gravity. I pulled mine up as high as possible and secured it to the wall to prevent syphoning.
The only piping under in the enclosed underbelly is the three tanks, waste tank drain pipes, FW tank overflow, the FW tank low point drain pump pickup (tee'd to the fw tank drain tube) and the two point drains tubes in the bathroom.
Let me know if you want any pictures.
