It sounds as though it is a flush valve. If the undercarriage is not covered, you should be able to trace the hose to see where it goes. It could also be that they re-routed the fresh water line to become the flush line.
It is normal nowadays to have a flush line installed on your black tank. We normally flush the tank using our flush line about once a week.
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Matt
2011 Chevy Suburban 2500 4x4
2015 Kodiak 300BHLS
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