That is a very good suggestion!!
On our boat, each tank has a ball valve at its outlet. If these valves are closed, the system would produce the sort of symptoms described.
However, the OP indicated that the system was working prior to his filling the tanks, so ????? Did he or someone else possibly close those valves when the tanks were filled?
I read it slightly different. I made the assumption some amount of time passed between when he viewed the boat and then bought it before he sailed it away. Given that time, the PO could have turned off valves as part of his standard exiting proceedure.
My other thought was that the initial test could have used up any water remaining in the line prior to the cut off valves.