The GPHS RTG contains 7.8 kilograms of plutonium 238 but masses 57 kg in total. It also generates only 300 watts from that 57 kg package:

https://en.wikipedia.org/wiki/GPHS-RTG

You'd need about 3 metric tons of them to power one Model 3 cruising at highway speed (assuming ~16 kilowatts continuous power draw).