Shower clog or something else?

I have an interesting problem that I have never seen before. During our last outing, I noticed there was standing water in the shower drain after the first shower was taken in the rig. I found it hard to believe that the grey water tank needed dumping already. So I opened all the valves to the tanks to drain all the water/wastewater. Still had standing water in the drain. I poured a bucket of hot water down, thinking maybe there was a clog and it would rinse it out. Interestingly enough, the water drained fast, but yet there was still standing water in the drain. We took another shower, and there was never any problem with a slow drain (the water drained fine and never backed up), but always standing water inside the drain.

Brought the rig home and drained out everything - grey, black, fresh, low-point drains, etc. Still standing water in drain. Used the vinegar & baking soda solution but it didn't help. Tried flushing it with the garden hose - nothing. Tried using that plastic hair drain tool - nothing. I am stumped as to why I can get all the water flushed out of the shower pipes.

After looking at the piping, it looks like there is no p-trap for the shower drain. Anyone have any suggestions as to what my problem is and how I can fix it?