Really, the only way to know for sure is to drop enough of the coroplast down so you can get eyes on everything. Before you do that, you might want to carefully drill a hole at the lowest point of the coroplast to let the water out gently instead of all at once when you are on your back on the ground under the tank. Watch out for pipe and wires. Hopefully (if it's not a water line leak) it's not your black tank.
Had that happen last year. Found the leak by filling the grey tank. Turned out to be a flange along the top of the tank that (of course) only leaked when it was full. I had to put in a new tank and put in a couple of extra support straps under it to take some of the weight off the flange when it was full. I also strapped the black tank as well since I was looking at it.
Hope this helps.
2014 Keystone Laredo 292RL
2013 Palomino Maverick 2902
2018 GMC 3500HD, 4x4, 6.5' box, SRW, Denali, Duramax, Andersen
DeeBee, JayBee, and Jed the Black Lab
The hurrier I go the behinder I get. (Lewis Carroll)