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When you open the transmission access door on the curb side of the coach behind the tag axle, you should see two quick-disconnect fittings in the upper RH corner. Those Hansen couplers are for filling the existing two tanks - the larger one for the chemical toilet's holding tank, the smaller one for the sink's water supply - which is gravity fed.
The toilet holding tank is right behind the couplers, with the drain mechanism below. The sink's holding tank is hidden inside the restroom behind the mirror, requiring mirror removal for service/access.
I think you'll find rather quickly that using the revenue service restroom's toilet is a real pain in an RV application. First of all, it's located on the wrong side of the coach for most dump stations! Other RV owners are not happy campers if you have to pull in bassackwards to dump. Secondly, they don't dump with a hose like an RV - it's a straight shot down when you pull the plug, splashing everywhere.
Revenue service restrooms are designed to be dumped frequently, depending on the operation. (A Greyhound might have it's dumped 4x in 24 hrs on a long-haul schedule, for example. A charter might only have to be dumped every other day, depending on use.) You'll also need to obtain some commercial coach toilet chemical to dump into the holding tank to control the odor, as the RV stuff is pretty weak, comparatively - but even this can drive you batty if it's not changed frequently. Passengers normally aren't bothered by this because there's a blower fan that runs constantly to exhaust the fumes whenever the coach master switch is on. This fan and it's duct work are also hidden behind the mirror, IIRC.
Not trying to burst your bladder here, just relaying some "real world" experience with revenue service coach restrooms in answer to your question.
FWIW & HTH. . .
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