Welcome! Oftyen a good place to get some answers----at times! But lots of guesses involved at times, too! In this case we get two drawings but not the best for showing exactly where the tank lies but we can kind of look at things to see what might be there for a rough idea?
You may also want to take a look for other things and this is the link to the parts catalog. Your RV falls into an oder form of drawing which is not as good as ome of the newer interactive stuff but we use what we get!
cortona.winnebago.com
Taking a small snip of the floorplan and adding what I think the plumbing drawing show, I get this estimate. they don't give us both sets of info on the same drawing, so consider some "sloppyness" in this!
It looks like the grey tank is forward of the black with pipe running where you might seeit to get past the black to the outlet! It looks ike the vanity sink drains pretty close down into the gry.
Then there is a space between the two tanks where the black front edge loos just forward of the stoool which dropps directly into the tank. It looks like the black tank is about centered behind the main dump drain.
I find nothing about the propane tank location, so no much help ion where you might be finding water leaking!
But one thing to mention is how tricky water can be in RV, as it tends to drip down and may run in any direction to get off the tip of things like tanks. that can change depending on the slope of the RV, so we can get different results every time we park. Be very slow to make any big move without checkiing for tricks!
Things like being sure it is in fact black water? What I found found is that I can have a leak that drips onsome flat surface to make a small puddle that doesn't come out until it do something to change the RV tilt! But a problem we usually have at the house! But we may have a leak one day that shows the next time we move the RV! UGH!
Makes me very slow to believe what I see at times!
Totally NOT saying you are wrong but just warning to test carefully before doing major work to fix it as it can be making us look like we are blind!