I have the Nuclear fuel mod installed and the Plutonium RTG is available through the creative chests, but uncraftable. No error messages of any kind show up anywhere.
In the pu238RTG.lua towards the bottom is the line "enabled = false,". Changing that to true fixes the issue somewhat:
It becomes craftable and will not show up if nuclear fuel is disabled. However it has no research associated to it and is craftable from the beginning.
Thought I should mention it.

Thanks for letting me know.
Seems like I forgot to add the pu rtg recipe to the technology when u235rtg was changed to be just a reskin and rename for fusion reactor and it's tech.