Plastic Potty to Porcelain Throne?

The bad news is that the riser ("box") is usually there to accommodate the drain plumbing into the black tank, or even the tank shape itself. Taller RV toilets have been around for many years, so when the coach builder uses the riser and short toilet, there's usually a good reason for it that's not easily changed. One mod I have seen though, is to remove the carpeting from the riser if you don't like it, and replace it with vinyl tiles or flooring.