Constant running on MY 2001 2900SCR's vacuflush head oftentimes was (and occasionally still IS) caused by "debris" between the ball valve's rubber seal and the bowl itself (toilet paper, waste, etc...) Because it's so unpredictable/quirky, I ended up installing my own "cut-switch" from back in the pump area (engine compartment). I installed it behind the transom shower door and when we leave Endless Summer for a few days, I always remember to switch the power OFF to the toilet with my home-made cuttoff system. To assist with the pump not always running, I've also resorted to having to replace the SeaLand/Dometic toilet ball valve ring assembly about once every three seasons. I guess they just wear out, and need to be replaced---I found mine at a local RV store, cost about $50. Seems that when water escapes from the bowl, the toilet "thinks" it's being flushed, hence the constant pump running.
But I'm with Steve/SP... why it wouldn't pump at all is a mystery. It's doubtful the motor wore out when you starved it of water, but I suppose possible.