Use some Oxyclean or PBW, soak about 2oz in five gallons of warm water overnight and then you won't have to do any scrubbing, actually it may take a day or two depending on how bad it is. But literally you can watch all the crap fall off the sides.
"Yes, I am a pirate two hundred years too late. The cannons don't thunder there's nothin' to plunder,
I'm an [under] forty victim of fate, Arriving too late, arriving too late."