Short of getting a plumbing diagram from Jayco, the only to know if something else is connected to your black tank is to test it directly.
If you are at a location with full hookups, fill and dump your black tank. Put a clear adapter between your hose and the outlet (we have one of these http://www.amzn.com/B000BUU5T0
that we use for the purpose). Leave the valve open and move around your MH turning on each faucet one-by one. If you see any new water running through the adapter, there's your culprit.
We have some friends who had a Coachman travel trailer with an outdoor kitchen sink that was plumbed to the black tank, so it wouldn't be out of the question.
I always use the adapter when dumping the tanks so I can know when the black tank is running clean while flushing.