I don't know, but I drove an RV for a month through Europe and we've never used the onboard toilet because we didn't want to have to deal with it. (We were also students on budget and wanted to avoid camps as much as possible.)

Of course I would prefer the RV toilet to a diaper though.

Buses with toilets probably have dedicated disposal facilities operated by the companies. Long-distance buses here are mostly run by big conglomerates.