Thanks - don't know why it didn't show up in my search.
That thread is about the drain piping (which has problems irrelevant to an Escape owner), and only incidentally mentions the venting/overflow, so I'll just Donna's word for it being an overflow at floor level... although it appears to be just a bypass between sections of vent piping, with no outlet. I only read 8 pages before skipping to the end, but the funniest/saddest thing I saw was that people are going directly from the Casita factory where they pick up their brand new trailer to a shop where they have the defective drain plumbing fixed.
I have to hand it to those Casita owners for willingness to do work on their trailers. Also, there are so many Casitas out there that there's a whole aftermarket industry to support them - sometimes there are advantages in numbers.