I can't help you with the GPS information, but will add this:
Not all of the river road is "great" I just did a couple hundred miles of it
(north out of Greenville MS) and it was flat/straight and boring,
mostly cotton fields and you "rarely" see the river.
I started out leisurely cruising but after an hour or so wound it up some,
when the MS highwaypatrol cars lights came on I was doing 72/55 or so he said,
(actually it was more like 75) I think he guessed because my Valentine never went off.
When he mentioned he had a Goldwing, he "instantly" became my best friend and we spent the next 30 minutes talking motorcycles, it worked,
he handed me back my license and said be careful.