Others have explained how power is not the issue, but also RTGs are not a cure-all. This whole sunshade thing is necessary to shield the sensitive electronics from the heat and radiation of the sun - putting an active radiation source on board would present huge problems for the same reasons.

RTGs are also very heavy and do not generate much power; they are useful for the outer solar system because solar power is not viable there due to the inverse square law. At this distance from the sun, solar is a no-brainer.