They stink, period. I've been in lots of campers with bathrooms and if you use them for number 2 the camper is going to smell at some point. There's a lot of plumbing beyond what you can scrub clean, and campers do sit for long periods. My simple solution is to use the camper bathroom for number 1 only, and take a walk to the campground vault toilet for number 2 (or when boondocking go dig a hole). I can't believe they put toilets in truck campers. 30 foot Airstream, ok fine maybe. Truck camper? No way.
On may last trip in southern CO I saw a Ford Transit camper van with a big metal rear tray. On the rear tray was a square-shaped structure, tall and big enough to stand in. It had a vent on top. I have a feeling I know what it was, and frankly not a bad idea.