To be clear, I wasn’t proposing hacking together a homemade RTG. That would be absurdly dangerous.
So of course, it’s something that probably ought to be possible, but I was thinking of, primarily, adding a big RTG already as a sealed unit which was destined for space or possibly the deep sea and/or a very remote location, until the cataclysm left it stranded on a truck or a railcar or something. In that case, all you’d need to do is plug and play, and if it was designed for deployment in, say, icy wilderness or the deep sea, it’s possible that it would have a signifiant heat output - intended to keep whatever it was installed in warm, but which you could hook up to a Stirling Cycle Generator for added power output.
Apropos of nothing, in the scenario which my friend and I were mooting but totally freeform our whole crazy idea was basically that a combination of railwaymen and literal rocket scientist/engineers had hacked together this super-monstrosity at a NASA railhead out of rolling stock and whatever satellite/space components they had on hand, thus resulting in a rolling cataclysm home that beats the pants off even Lydia the Fluffy Tentacle Mutant’s Doomtrukk, and really puts the English on the phrase “flex fuel,” what with having a conventional diesel-electric locomotive, solars, an RTG and a Stirling Cycle Generator (which itself made use of waste heat from the diesel-electric, the RTG, and could be directly fueled by the simple expedient of a firebox,) able to provide both electrical and motive power. Sadly, CDDA is not equipped to model such, we were more just thought experimenting.