You assume RTG technology - I don't and I don't think the linked article does, either.
They discuss Energy Organs here, stating that (emphasis mine):
a sphere of Gd148 emitting ~100 watts with a 75-year half-life and measuring 3.41 cm in diameter with a 5-micron Pt shield glows at 1326 K (e-sub-r for Pt at 1326 K is 0.156; Gd melting point ~1585 K, Pt melting point ~2042 K); this is approximately the decomposition temperature of diamond (into graphite) and well above the combustion point for diamond in air (Section 6.5.3), so Pt-coated sapphire (sapphire melting point ~2310 K) may provide a more stable first wall for the radionuclide energy organ. Carnot thermal efficiency for a heat engine using this source could reach, at most, ~76%.
I'd say that's pretty good efficiency, and given the power levels and temperatures, I think non-RTG technologies should be used. If the system never drops below 0C, why not use a more conventional system?
Plus, you could just use the Gd148 to keep the craft warm and use other means to generate electrical power.