For more efficiency than an RTG, slap a Stirling engine on the heat source (https://en.wikipedia.org/wiki/Stirling_engine). Probably not practical, since the necessary radiators would be huge and it introduces mechanical complexity.
But definitely a large increase in complexity as Stirling engines are mechanical devices and thermoelectrics are solid-state. The Stirling engines can be over 25% efficient rather than like 6% for thermoelectrics.
NASA has operated a Stirling engine in a lab for 17 years so it’s feasible, at least.