I would connect, turn it on and watch for a leak in the area under the seat where the tank used to be. And regardless, for redundancy, look for the old connections and cap them off, if the PO had not done it himself. The check valve and pump may not be absolutely reliable. My fresh water tank was once filling up from the city water, but only once. Maybe some debris was in the system causing the backflow.
Why would the PO not have simply cleaned and chlorinated the tank - that is a question.
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