A number of years ago I purchased a used camper and I soon discovered that lingering "toilet smell".
I blasted out the holding tank, repeatedly! Checked the stack to be sure no nests or other blockages where the culprit. I scoured the toilet bowl. EVERYTHING I could think of.
The smell got a little less noticeable but it was STILL THERE.
A couple years later, I removed the toilet because I would be dry camping for a season and the toilet space was used for a portable camp toilet.
I brought the camper toilet home with the intention of storing it in the rafters of my garage. It never quite got there.
I was working in the area of my garage when I suddenly got a wife of that all too familiar odor - the camper toilet!!!! It was that darn toilet the whole time.
No more plastic for me - 'nuf said.