Grey Water Tank is draining really slow now....

I had a problem like that with a gray tank and it had nothing to do with what I had put in it. It had everything to do with what somebody at the factory left in it. I removed the blade valve (which was mounted close to the exit from the tank) and spotted a tank cutout wedged in the exit pipe. I was able to remove that with a pair of needle nose pliers and haven't had any problems since.