Debi,

This is an issue that is not hard to visualize. If you have a HeadHunter type toilet with the associated flush valve picture in your mind a toilet bowl with water in it. The water in the bowl is in direct contact with the waste line going to the black water holding tank, and it is also in direct contact with the fresh water used for the flush cycle.

What separates that toilet bowl water from the fresh water holding tank is the flush valve that under almost all circumstances seals so tight, it shuts off fresh water that is under pressure from leaking past the valve. That seal also prevents any water in the toilet from backflowing into the holding tank.

It is only when the flush valve fails to seal, or shut off the flow, that problems occur. In any coach in which the flush valve has functioned properly it is unlikely the fresh water holding tank has been soiled. In fact, in some cases the converter has taken steps which prevent the contamination of the fresh water holding tank by installing a check valve between the pump and the flush valve, thus making contamination even more unlikely.

When we started "camping" in our first coach our concerns were like yours and we just decided to never ingest water from our holding tank, instead preferring to use bottled water. Even when I modify our plumbing system to almost completely eliminate cross contamination we are unlikely to change our practices.

In the context of the toilets and valves discussed above an air gap is not possible. It would require the valve to be mounted above the toilet, and to allow the water to flow into the toilet from above the water flood level. The way the toilet is designed it requires the high volume flow into the waste line below the bowl to create a very low pressure area to "vacuum" out the contents of the bowl. It is a very effective design, but like I said originally, it is a compromise.