After two buts and two briskets on my 85g I had the same thing...lotsa greas in the bottom. I didn't want it going rancid on me so I poured in some Heavy Duty Simple Green (which for some reason is purple) and maybe a half gallon of water. I swished that around and let it soak in to break up the grease.
I poured this out into a 5g bucket and then down the sewer drain. I used a hose to rinse out the rest with the drum tipped on its side. I mostly left the sides of the drum alone.
After that a quick blast with the weed burner to dry it out and a quick coating with some Smart & Final spray grease (like PAM) and we are good as new.