Since you already have the holding tank, vents, etc. all intact, I would replace the toilet.
Rather than replace with an RV style, I would use a yacht type. They look almost the same, but have a mascerator pump. The expensive ones use an electric pump, the less expensive ones have a handle on the left side. Pulling up on the handle pumps water into the bowl and pushing down pumps water and effluent out.
The advantage is the solids are mascerated (made into a slurry) before going into the holding tank, and are pumped. This allows a holding tank to be somewhat above the toilet. So you could possibly go to a larger holding tank, if desired. Also the slurry is much easier to chemically treat and dump.
Much info on this is found by Googling 'boondocking' and getting on the Phred Phrannie poop sheet site. Good info on a lot of boondocking
subjects. I highly recomment this site.