According to this item from the Cassini website, Cassini's RTGs will generate 628 watts at end of life (11 years after launch). I take that to be the combined output of all of the RTGs. Together, those RTGs use about 70 pounds of Pu-238 plus a lot of other stuff for safety.

By way of comparison, the ion engine (see page vii) on the DS-1 required from 423 to 2288 watts to operate. The 2.3 KW figure was necessary to generate 92.7 mN of thrust. All of the Cassini RTGs put together would only allow the DS-1 ion engine to produce about 22-25 mN of thrust. To put that in perspective, the full thrust produced by that engine is roughly equivalent to the force imparted by a sheet of paper held on your fingertips.

For more info on the Cassini RTGs, you can start here.

The short answer is yes, you could use RTGs to power an ion engine for a deep space mission. However, it'd take about 300 pounds of Pu-238 and a lot of other mass to fully power an ion engine like used on DS-1. The Greens would really freak at that number. By the time you added the rest of the RTG mass, you'd probably be looking at something like 600-800 pounds just for the RTGs. Add the propellant and ion engine mass (DS-1 numbers were 125 kg of propellant and 8.33 kg for the ion engine) and you're likely going to be too heavy to launch on a Delta II booster. That means your launch costs will be a lot higher, too.