Sounds like another dry/flush valve failure
There have been a bunch of those recently in Jabsco manual toilets. Only other possible explanations are: intake seacock not open (you eliminated that one), grit in the pump that's preventing it from priming, an air leak in the intake hose that's preventing it from priming, or it's been so long since the toilet's been rebuilt that the the seals in the pump are so worn that the the toilet pump can't seal any more. However, that last one would also affect the toilet's ability to move waste out of the bowl as well as its ability to draw up flush water. So I'm betting on the dry/flush valve. Call Jabsco for a replacement.
