Help me identify my Leak! I'm a newby.

[fatcatzzz]

Looking at your floor plan looks like that pipe is from the washer hookups. Not uncommon
for the washer drain to go direct into main drain (3"). I see that the toilet is as so on the other side of the wall and the black tank is there. Black tank is most likely the problem and just leaking through the opening for the smaller drain. Unfortunately only real way to tell is drop the chloroplast, I had a leak on my 07' Montana that ran about 6 feet on top of the chloroplast before it came out. Good luck.