Can we rule out an RTG, given the half life of Iodine 131 is about 8 days, and would seem pretty pointless to use it for space stuff.
RTGs don't run a controlled fission chain reaction like a nuclear reactor does, so there is no means by which they can evolve I131 (at least not in large quantities). So yes, I think we can rule out an RTG.
Plutonium is weird stuff. A lump of it will crack and fall apart, because it's so radioactive that parts of it transmute to different elements which then disrupt the crystaline lattice of the metallic plutonium. It's normally used as plutonium oxide powder (or pellets of the same), which is much more mechanically stable.
RTGs are simply a large thermocouple heated by the plutonium.
AFAIK the effective limit on their lifetime is a function of the damage (transmitation) caused to the thermocouple by the radiation coming off the plutonium.