I think a few factors combined, may have kept the tow truck driver from smelling anything out of the ordinary.
1. His sense of smell was off due to a cold.
2. By backing the tow truck up to the rear of KC's car, the exhaust from the truck may have temporarily overwhelmed the smell in the trunk.
3. By looking at his time line, he was only at the rear of the car for a very short amount of time, for a quick hook.
4. Not sure what the weather was that day, but wind direction, if there was any, could have also played a role. Wind may have blown the scent away from him.
5. He may not have paid attention to any smell coming from the car, due to the fact it was parked next to a dumpster.
I still believe in the original timeline of June 16th, being the last day of little Caylee's life. Nobody has come forward claiming to have seen her after June 15th, other than GA who claims to have seen her in the afternoon of the 16th. Just because 1 tow truck driver claims he didn't smell anything on the day he picked the car up, doesn't mean the smell was not there IMO.