Well, I think they do, but at such a low rate that they effectively don't. In my current playthrough I had an RTG installed for 3 sols now, and its slot still didn't drop 1% during that time. At this rate, they practically don't require maintenance ever, since even at a moderate starting condition they'd last 100-150 sols before breaking.
On a marginally related note, solar panel mounts also degrade very slowly, maybe at half/third of the rate of the other component slots, but it's at least somewhat noticeable, unlike the RTG.
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