LETTER TO HEAVEN
(Dolly Parton)
« © '69 Velvet Apple Music, BMI »

That is from

www.luma-electronic.cz/lp/p/Parton/dolly_joshua.htm


There are other lyrics from the album Joshua, and they have various writers/authors attributed. So, I would say that Dolly is indeed the author of that one.

As to whether or not she based it on anything else, I would suggest you e-mail her. She's a down-home gal, and more than likely to answer you.