I don't think I've ever seen a gas C with the generator mounted above the floor line. That would effectively put it inside the house, even if boxed off. It is a strange checklist item for a C, but might apply to other RV types.
I've seen big diesel generators located center-forward above the chassis rails in rear-engine diesels, some Sprinter Cs supposedly have engine compartment generators, and fivers sometimes have generators in the compartments under the bedroom. Conceivably you could put one in the "garage" area of a toy hauler if that did not double as living space.
They get put where there is space to put them isolated from the living area. The above frame mounting location in the case of large diesel pushers is because those generators are big and heavy.
Tom Test
Itasca Spirit 29B