RVIA does not seem to have a requirement for separate tanks. But several years ago, there was a move to require separate drains for black and gray tanks on RVs. I suspect that the push originated when new rules for wastewater treatment came into effect. Rural campgrounds, with no connection to city sewers, realized that treating gray water was a lot less expensive than treating black water, so if they could keep them separate, it would benefit them. It is also possible that filtered gray water could be used for certain purposes with little or no treatment - think golf courses, crop watering, car and truck washing, and so forth. A number of trailers, including a few TMs, were built with separate outlets.
The push for separate RV drains didn't last, but it did happen at about the same time as the formaldehyde scare, so they were possibly related.
Bill
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