Easiest solution will be to run some hot water down through the drain and let it warm up the grease enough to drain it. A couple big pots on the stove on high along with letting the water heater get nice and hot. Then turn on the hot faucet until it runs out of hot water, dump the big pots of boiling water down the drain and give it about 10 minutes. Drain the gray tank and repeat as needed if you don't see what you think is normal draining.
Going forward, We wipe anything that has grease down with paper towels first, then wash just the residue off in sink. If it's a real greasy item (;ile a pork chop or rib pan, it's gets the hot water in a tub outside treatment.
