On our 2015 Rockwood 2318G the vent for the shower drain is tucked away at the bottom of the cabinet that is between the shower and the front bunk. The “floor” of the cabinet is not flush with the floor of the trailer, leaving room for the vent. Yours might be in a similar place. Good luck.
Wow, my 2007 Starcraft 2406 had the shower vent on the exterior of the body. I'll see if I can post a pic.
You can see it next to the toilet cassette door, that's at the rear of the side wall. The vent is to the right of the toilet cassette door and just under the stove on the side wall.