How much time is allowed before discovery?
The room has a sink made of a material that can take a good amount of heat. Stainless steel comes to mind but there very well might be other materials that aren't coming to mind at the moment.
After doing his dirty deed the killer climbs onto a contraption sitting in the sink. (This will be a bit precarious due to the narrow base it must have. There could be additional supports while he was getting in that are then removed.) He's lying on a membrane separating himself from a mixture of sulfuric acid and hydrogen peroxide. After he's dead a timer triggers, making a hole. In time the chemicals dissolve the body. (Yes, this is basically a scene from Breaking Bad. The show didn't give the details and it wouldn't have worked exactly as shown anyway.)
After enough time has elapsed a second timer opens a drain line which is run down the drain past the metal parts. Away goes your chemicals and all obvious traces of the body. Removing all that weight causes the device to fold up (and pulls up the drain line) and triggers the next stage--the release of a solvent for the plastic of the main part of the machine. The last piece to melt through is another drain--that part is now gone.
Yet more time, a container of water ruptures, washing the gunk out of the trap. At this point you have some scaffolding, the bowl that held the melting plastic and whatever held the water. (The timers are chemical in nature, no electronics are involved.) These are all built out of something that will burn slowly and cleanly, the last stage is to burn it. (This timer might be as simple as something that will spontaneously ignite once dry--but it was in the water holder.)
There's no such thing as truly zero residue from burning a solid, there will be traces left but not much. A disguise of a controlled kitchen fire earlier would make it even harder to find. (Something on the stove burnt in a situation that kept the size of the fire down. Lots of smoke but no fire damage. Leave the burnt stuff as evidence of the nature of the fire.)