RTGs have other issues as well, they need to radiate all heat on the "cold" side into space, so you're between a rock (high hot-cold temperature difference need

RTGs have other issues as well, they need to radiate all heat on the "cold" side into space, so you're between a rock (high hot-cold temperature difference needed) and a hard place (need high cold temperature to radiate) even before you get to thermo-electric generators being inefficient.