I'm not entirely certain I'm understanding your question and I've never heard of "The Rule". However, it seems that you're asking about the handling of 'solid waste' aboard a bus and if that's the case the answer is 'no problem'. Every "self-contained" RV in existence has a holding tank to address that issue. On most rigs the toilet sits directly above the waste tank and gravity (along with water coming into the toilet bowl) takes care of getting the waste into the holding tank. The holding tank is emptied at an RV disposal site (State Parks, some Rest Areas, KOA's, etc) by connecting a hose to the outlet of the tank and runing it to the sewer hookup (usually a hole in the ground). Then you open the outlet valve on the tank to empty its contents.
1982 Thomas Saf-T-Liner Pusher "Illusion"