2018 16TBS drains shower into black tank....

[ADR]

Can't believe they did that...only the puny kitchen sink drains into the grey tank.
Might as well just had one huge tank for both.

Which has me thinking-  anyone investigated a cross connect with valve to transfer black to grey if it gets full?
Can't see up under there due to the so called "enclosed heated underbelly".
Prepping for a 30 day+ trip so may not have time to work it out before but it will definitely be on the "to do" list when I get home.

[Merlin]

Really? The 16 too? Yuck.

[ADR]

Yep....couple that with the wasted pint of water on every flush of the toilet- and the black fills really fast.

I'm wondering if they have a check valve in the shower drain-  if not guess where an overly full black tank rears its head? 
Yep in the shower

[Paul]

@Gary M had the same problem with is 21BHS which he solved. You can see the following thread https://aluminumcamperforum.com/index.php?topic=889.0

[Gary M]

There is a check valve under the shower(the circle looking thing in the picture. I capped it off and ran my own drain into the gray. If you do it the way I did, be very careful drilling your initial hole thru the floor. The tank sits right up against it and it's easy to accidentally drill thru the tank.

[Gary M]

I used a 3/4" blind bulkhead fitting which was the biggest I could find. The hole in the floor needed to be bigger than the hole in the tank so that I could fit tools in there to tighten the fitting. I used a 4" holesaw for the initial hole in the floor and it turned out to be just the right size. Any smaller and I probably wouldn't have been able to tighten the fitting. Had to use a bonney wrench too.

[ADR]

There is a check valve under the shower(the circle looking thing in the picture. I capped it off and ran my own drain into the gray. If you do it the way I did, be very careful drilling your initial hole thru the floor. The tank sits right up against it and it’s easy to accidentally drill thru the tank.

I'm hoping there is a check valve- when I tested where the shower water went I dumped about 3 gallons into the shower pan it drained very slowly.  Hoping that is because of a check valve.
I'd love to dig into this right away but I will wait until we get back from our trip- can't risk messing something up and not getting it straightened out in time.

Your previous pioneering in fixing this will be useful I'm sure

[Gary M]

Oh yea take your time and plan it. It's definitely not a fun project, although you will be glad you did it once it's finished 👍🏻.