A simpler (and unfortunately more expensive) option may be to replace the toilet with a composting toilet. Options include: C-Head: https://www.c-head.com/
or Air Head: https://airheadtoilet.com/
or Nature's head: https://natureshead.net/
On the plus side: No plumbing is required. No cutting fiberglass. No adding a black water tank. No vent pipe so no cutting holes in the roof. No having to visit a dumping station to clean out the black water tank.
We're thinking about replacing our standard toilet (same set up as shown in the photo) with a C-Head toilet in our Oxygen
. The difference is using the C-Head we can route the "liquid" to the black water tank underneath using a Bottom-Exit-Kit: https://www.c-head.com/accessories
, instead of using a removable container. I understand you can dispose of the contents in any vault toilet at most parks.
Just a thought.