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Most modern RV's have separate grey and blackwater tanks, but many older units (including the 1973 GMC RV) had a single combined tank.

You sometimes still still a combined tank on modern units, usually smaller ones.

I don't know what you mean by "chemical toilets", I put digesting/deodorizing chemicals in my RV's black tank, does that make it a "chemical toilet"? Either way, the liquid I drain from the black tank is definitely what I would call "sewage".

Black water (from the toilet) is most definitely sewage. Where did you get the idea that it isn’t? Grey water can be sewage, too.

My RV doesn't have separate tanks or a chemical toilet.

Some RVs have separate black water tanks (toilet specifically) and grey water tanks (shower, sinks). Not all do. Sewage is definitely black water and not grey.

Considering people take a leak in the shower and stuck excrement can get lose during showering I would argue gray water has various shades of gray up to black.