As far as I know there has never been an RTG powered electric rocket engine. RTGs are extremely expensive per unit of power output. Spacecraft like that ion engine propelled one that was sent to Ceres use solar power.

Voyager and Pioneer used RTGs, as well as some more recent longer range missions and the planned Dragonfly to Titan. Agree that solar has much better unit economics, but I don't see any reason why the same wouldn't be true of fusion too.

And none of those use RTGs to power electric rocket engines.

(EDIT: thrusters are rocket engines, just very small ones. All those missions used small chemical rockets for maneuvering, using storable propellants like hydrazine.)

Edit: I stand corrected on Voyager/Pioneer exclusively using hydrazine, though you could get similar thrust from a modern HET without needing insane amounts of RTG power, and run other electrical equipment off that extra power when you've reached target velocity and aren't firing the thrusters for attitude control. The higher Isp of the EP system is an advantage too...