Much later than I had meant to write about this, but … let’s talk PLUMBING…
I’ve had an issue with water backing up into my shower instead of flowing into the drainage pit. I called many plumbers/handymen but no one would work on any structure they weren’t insured for – the inbetween tiny didn’t qualify as an RV or a house. BUT, I was saved by an old friend from high school who is now a plumber. He graciously came on his own time and brilliantly rearranged the pipes. Now, things are smooth as can be.
Unfortunately, though, I haven’t been able to consistently use my shower since living in the tiny (I’ve adjusted – showering at the university gym or a family member’s house). Two central problems:
1) Because it’s a wet bath, I step into the shower every time I use the bathroom. This leads to debris (and, in this house, cat hair), which eventually clogs the drain. I now keep the drain plugged and clean the shower out before showering. And I flush the drain once a month with baking soda, vinegar, and boiling water.
2) The entire wet bath is pieced together from old aluminum ducting. Because the ducting is in strips, there are seams that have inevitably leaked. That, combined with a custom drain that is ill fitting, led to a few water bails-outs from under the sink.
– I tried sealing it all with marine grade sealant, but the sealant yellows and catches dust/hair. It was gross.
– I removed the sealant and instead primed/painted it with this (which supposedly fills seams):
After three coats, it looked solid. And it seemed to work – for the first week or so. Then the seams began to break and a very small amount of water trapped between the shower and the tank below it. Because I haven’t had the time to fix it, I haven’t showered in it since. Soon, I will try a different sealant for the big seams, and then 5 to 10 coats of the elastomeric coating over it all. Fingers crossed that I can shower here before the new year. 🙂
In the meantime, happyhappyhappy holidays from the tiny house! We are cozied up and hoping you are too.
aatinyhouse 
You may want to use the product “Redgard”, it goes on pink and dries red, so you’d have to paint it after. But, I would think it would seal the best. Cheers!
GREAT TIP – thanks, Tim!