Photomike, What I suggest may take a bit of a change in prospective on how to solve your problem. I would recommend that you research the dry chemical route. I am talking about the system developed by NASA and used in camping. It works basically by you putting a large bag below a toilet seat, do your business, seal it up and place in a sealed ziplock. You then can legally dispose of it in the trash. The chemical powder in the package neutralizes the material and makes it safe for disposal. We have a setup and use it when camping in an area where you can not just bury your waste. They are used in the military and fire camps when needed for remote locations.
With the truck camper setup, you can just buy the individual use packages and place the liner in your existing toilet, no need for their toilet seat. Do your business, then seal it up and dispose. No more smell than normal use, and you will not mess up the toilet.
May not sound easy, but it is and really has no risk of soiling yourself as you fold it up and put in ziplock. Take look at the link below, other companies sell and ours came from Cabala's. The company below has a free sample of one kit to try,
Seams a lot simpler than removing a toilet just for winter use.
Clicky