Trace the vent line from the tank to the hull
It's the "little chrome piece," but so are the vent thru-hulls for your water and fuel tanks...you need to check the right one. (Actually, you should all of 'em, regularly--to prevent blockages). If it's the kind that looks like a cap, it has a slit in the bottom of it...you won't be able to look through it. Clean it out with a screw driver. If that doesn't free it up, you'll have to take the vent hose off the thru-hull--and maybe off the tank too--to find the blockage.You won't be able to pump out until you clear it. However, unless the macerator impeller has fried, you can dump the tank at sea if you open the deck pumpout fitting to let some air into the tank. If the tank vent IS blocked, the tank is likely to be pressurized...so be prepared for an eruption. Open it very slowly, hang on tightly to the cap, and make sure you're upwind with a hose at the ready to clean up the deck. If I were you, I'd relieve the pressure before starting work to clear the vent line.However, you just gave me another major clue: the macerator motor is overheating, blowing its internal breaker. That indicates a clog in the macerator....that there's something in it that has the impeller jammed, preventing it from spinning, and is overheating the motor. But before taking it apart, I'd make VERY sure that a blocked tank vent isn't causing the problem...and then pump out and THOROUGHLY rinse out the system.