Dry friction heat destroys impellers in all pumps...which is why impeller pumps should not be run "till dry," but only as long as any liquid is passing through 'em. So you don't need to keep replacing the pump, just the impeller. However, it doesn't take many impellers to add up the price of a new pump.

You say you have no way of knowing when the tank is empty...maybe, maybe not. Even if you don't have a reliable gauge, if you can hear it running, you'll hear it change sound when the tank is empty. Or...you should know how many gallons should be in the tank...a macerator pump pulls about 12 GPM through it...so time it. There's no requirement to dump every last drop in the tank, so your time estimate doesn't have to be exact, just a conservative approximation.

If you can't even manage that, then a diaphragm pump is prob'ly the best solution for you. The SeaLand (now Dometic or Dometic/SeaLand) "SaniPump," aka T-Pump, is the best. http://www.sealandtechnology.com/pdf/600340678%20Discharge%20pump%20instr.pdf It can run dry quite a while--though NOT indefinitely!--without damage. The thru-hull issue is easy to solve with male- male 1.5" x 1" reducing adapter.