Okay! Sounds like there are some good and some bad involved.
One good one that I "feel" is correct is that the black water holding tank is not the first thing to freeze for a couple reasons. One is the fresh water lines are much smaller and in places that get cold quicker. No need to tell a fellow from your part of the world about that stuff!
But the stuff in the black tank also freezes slower and the space is pretty wide open and broad so it seems like it would take far more to freeze solid enough to break? Some guessing there, for sure.
But it is easy for water to drop out on the tank top and run around before getting down to the bottom, so just suspect sneaky things like that.
So if you were in the Rv and traveling, you likely kept it warm enough not to freeze the tank and fresh or grey freeze first? But putting water directly into the black and finding it leaking is a good test.
But I'm still on the idea that it is likely to be a fitting, etc. that leads to the tank, not the tank. That would be good as getting to the tank can be a bear! They are often dropped into a specific built space as the thing is put together, making it hard to get out without rebuilding the RV.
For info, this seems to be the right parts drawing for you and it might show some things that "might/ could" lead you up a blind alley.
https://www.winnebago.com/Files/File...p_plumbing.pdf
Page two shows the grommet for the stool and vent pipe which goes out the roof. Those can get dry and fall apart or leak if we get the tank too full, so check and hope it might be happening? Easier to get to than the whole tank!
Then page four gives some info on the drain connection. I is at the tank bottom and so will leak with less water in the tank than the stool connection, so we can sort of use that to sort which might be the problem. Stool or vent only leak when really full or flush the stool, where the drain might be expected to leak with only half tank full?
I'm likely telling you things you might already know but just my way of trying to sort the problem. I think that has an official name of "mansplaining". Hope that's not offensive to you!
This is from the section on plumbing and gives some other details:
https://www.winnebago.com/Files/File...j35p_plumb.pdf