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To the best of my knowledge that vintage Liberty had manual dump valves. Later models have air operated dump valves but controlled by a touch screen device in the inside of the coach if a password is used, or by buttons at the holding tank bay.
A Liberty holding tank is in the last bay, driver’s side adjacent to the generator. From the bottom of the tank is the dump valve, an elbow extending down and out and the sewer hose connection fitting just outside below the floor
Some Liberty coaches also had an optional macerator mounted on the top of the holding tank, fed from a smaller outlet pipe at the bottom of the holding tank, going up and over the tank to the macerator.
But like I said previously if a previous owner made modifications you have a do it yourself problem to solve.
Comparing a previous RV regardless of make to any Prevost conversion and finding they are set up and have the same systems is unlikely.