I want to reduce the speed of a basic remote control car so that it takes the wheels anywhere between 15 seconds and 30 seconds to make one revolution. Can this be done using a pot inside the receiver section?
Even with more circuitry, you would get a very erratic rotation.
A step motor would be just the right thing, but I sense it that you want something that's more or less done(?)
A DC motor with gears (worm gear, planetary gear or spur gears) can be had in a kit form, where you select the gear ratio. You might need two gear sets to get it as slow as you like.
The DC motor, if it have the same parameters as the one in your car, can be mounted (electrically) as a substitute for the one in the car, if you want to use the wireless control.