Gibb, the extra "r" in (some) English "dialect spellings" does usually have a meaning. It means that the preceding vowel is sustained or the syllable is emphatic.

Christopher Robin said, "He's Winnie-ther-Pooh. Don't you know what _ther_ means?"

An American would likely have written "Winnie-THE-Pooh" and "what THE means."

And I agree it would take some doing to sing "Rolling John" to the "Blow, Boys, Blow" tune. Stanzas from "Blow, Boys, Blow" (if indeed that's where they originated) and "Sally Brown" were among the most ubiquitous "floaters" of all, so my guess is that they got stuck into "Rolling John" arbitrarily.

If "RJ" was common enough to be reported independently three times (the "Maine" version may ultimately have come from Clark's book), I suspect the tune may be findable. Will look a little further.