Keeping a black toilet clean can be problematic. They not only can get a white ring on the inside of the bowl from minerals in the water but they also show dust. Try vinegar for the inside otherwise you can always have it replaced with a white or bone colored toilet.
I know this is a late reply to your question but we have one in our 92 coach and since I have upgraded the water filters on the tank we do not have the hard water deposits that people speak of experiencing. We also use vinegar and natural enzymes once monthly which helps this process as well.
When we purchased the coach the previous owner was using the Everpure ADC replacement cartridges but we have been using the Everpure OW2 filters which are a little better product which yields a more pure water with even less sediments to get trapped in the black sink and toilets. Just an idea, I'm sure there are many.....have fun!