An RTG uses the Seebeck effect, which gets you microvolts per degree kelvin. A Stirling engine can theoretically extract many times more energy from the same temperature differential, but with a larger complexity.
Actually, when I mentioned thermoelectric conversion I wasn't referring to an RTG (which is just nuclear decay) but rather to the true nuclear reactors that the Soviets put in space:
https://en.wikipedia.org/wiki/Romashka_reactor https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor
They used thermoelectric conversion rather than a Sterling engine with moving parts (although I guess TOPAZ is technically "thermionc coversion").