A few other options
What about a maceration toilet. I am not too familiar, but they apparently burn the waste.
Another option is that some park model trailers come without holding tanks. They are set up to be on long term lots. Again, I am not sure about the plumbing set ups, but may be worth investigating.
A third option is to keep the rv toilet, but put in a long term connection to your septic tank and put in a mechanized valve where she would only have to push a button to empty it. At her age, foot mounted flushing may be a problem. Also, she would still have to add treatment after dumping the tank.
It's not just the black water, but the gray tanks you have to consider as well.