On the later model RV, like you have, Winnebago has given us a really great tool to look inside and through the walls to see where parts are placed and what we might find where:
https://catalog3d.winnebagoind.com/menu/Parts.htm
It takes a small download but when dealing with questions, it can give a real help as we can turn things over and around to spot details.
Looking at the parts drawings, I spot something somewhat odd on your RV as the shower and sink go to the grey water tank but the vanity sink goes into the black! Not the normal thing so perhaps something to be aware of for any full tank questions?
I marked the grey tank as yellow and the black is left as purple and I did a few markings that I might see helpful. One big thing on your question is that the only venting is up and out the roof, so there should be no question of it overflowing for this leak as it is more likely to come up in the shower floor if totally, totally full.
But when turning it over to look at the bottom at the drains, there is a big suspect as it seems to show a rubber seal/fitting at the drains and those are normally installed with ,metal screw type metal bands which can get loose pretty easy!
So can you spot a place to get a look at that area and see if there is a rubber fitting with a seal that needs a bit of tightening?