Hi mk216k,
Sorry I missed seeing your post sooner. I saw it when I saw Ninelitetrip had posted and I hopped in to check the thread. I'm not sure I follow your question regarding the shower drain. The black shower pan drains via the blue hose shown in the picture. The shower pan hinges up to reveal the battery box underneath it which is when you can see the blue drain hose route from the bottom of the white panel out the rear of the camper. I think what you may be describing as the "rectangular black trough" is the battery box/holder. No water flows below that upper drain pan. All water flows out the drain pan hose and out the exterior drain. I think this may be getting redesigned (hoping anyway) to move the batteries to the passenger side rear compartment where the inverter is shown in my pictures. I believe the intent is lowering the height of the shower pan.
What I would like to see is the REDARC, Inverter, and batteries all in that exterior compartment at the rear passenger side. It would keep the REDARC close to the batteries (which is important) and allow a lot of room for batteries of different sizes. And it's important for inverters to be close to the house batteries, too, even with large gauge cabling. The DC cabling from the pickup could still run through that compartment over to the REDARC as could the cables from the solar panels. This would also make the compartment under the drivers side dinette seat (the one with the Truma AuquaGo and VarioHeat, water pump, and water tank) much less congested and easier to service. True that relocating batteries and the REDARC to that compartment would eat up some storage space, but small trade off, in my opinion, for what you would gain; The capacity for a much larger battery bank if the owner so chooses, and, for everyone, increased indoor shower headroom. I'd like to see the showerpan right down as low as it could possibly go to give reasonable head room in the shower. Since one would still want foot/leg support when sitting at the dinette, the lid over the lowered shower pan would be raised to about the height of the current shower grate. That lid could be hinged or lift-away and would give a handy place for storage when the shower is not in use and would allow a nice floor finish where you put your feet rather than feet going on a shower grate all the time. Basically, flip flopping what is in the model pictured; the shower pan is in the basement and a nice lid to rest your feet and what you see the majority of the time (when not using the shower).