So how's the experience level on bathroom plumbing? Got an idea of what the wax ring under the stool does, etc?
My thinking is this. The tank itself is probably not the cause as you see it on top but something above that. Winnebago does a really good job of posting online info for the later model motorhomes and you'v done a good job giving enough fo the important info to let me look at the parts drawings for your rig.
I'm seeing no other openings along that top and the line drops straight down into the black water tank. so my move would be to pull the stool and check the big ring that seals the stool to the floor and passes the waste on down passing through the floor.
Hoping to see a puddle and make it possible to replace whatever an RV has that functions the same as a wax ring on house plumbing!
Hope I can get a good enough picture posted to make it clear to you! Yellow is the black tank, green and blue are the area I mean? Guessing but this might be a simple hardware store item??? Pull the stool to see more!
From the parts catalog:
2015 Winnebago Vista 31KE on 2014 chassis