My holding tank is almost half full. When I went to empty it offshore last weekend the pump ran but pumped nothing overboard. It was running dry
10-1 odds that your tank vent is blocked, causing the pump to pull a vacuum. Clearing the vent should solve the problem. I wouldn't use the toilet again till you do, 'cuz flushing against a blocked tank vent pressurizes the tank and plumbing, which can have some very nasty consequences.
I hate to disagree with the mistress of waste , but my guess would be a macerator or discharge pump located dry above the waste that has a hunk of something that keeps it from pumping.
Depending on the pump location (height vs waste in box) simply filling the holding tank thru the deck fitting with a pail should get it working.
Most discharge pumps are very poor (Obendorfer and Galley Maid excluded) at suction and a tiny scrap will stop the lift.