Hello again! I use a Weber Jumbo Joe to barbecue, and to help clean the 18" grate I use a 20" water heater drain pan with the hole in the side sealed with duct tape and lined with foil (optional). 
I put very hot water in it, with vinegar and lots of dish soap, and soak the grate in it when I am done grilling. An hour or so later, the gunk scapes right off. Do this near a sink or outside though, a 20" shallow tray of hot water is hard to handle/carry. 
