I would call Grady if no one responds. I assume that the head will flush OK but not the macerator that you use for overboard discharge ( in legal waters
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That macerator usually is located in one of three different general areas; In the holding tank itself, on top or very close to the holding tank, or close to the place where the overboard seacock is located. I am generalizing since I don't know for certain.
On some macerator pumps, there is a access hole on the end of the motor that may have a plate screwed over it that when the plate is moved out of the way , you access the shaft of the motor and using a screw driver, turn it back and fort to unclog the impeller.
If your fuse blows immediately when you hit the discharge switch, a clogged impeller may not be the cause. Usually it takes a bit of time for the fuse to pop.