The question was asked:

Do the RTGs provide charge current for recharging batteries or are RTGs the sole source of vehicle electrical power?

The RTGs previously used have always provided direct power without chemical batteries. The concept of the RTG is really quite simple. The plutonium is always thermally hot. A semiconductor P/N junction is put between the plutonium and a radiator. A steady DC current comes off the P/N junction due to the heat difference. The life of the RTG is limited by the amount of radiation damage the P/N junction can tolerate. This is about 20-30 years. Since there are no moving parts, RTGs are almost fail safe.

Gary