Your question was very interesting to me, so I did a bit of investigation from this end. I liveaboard near the confluence of the Oklawaha River and the The St. John's. River and near the entrance of the Cross Florida Barge Canal.
Harris, Dora and Eustis are parts of what is known as "The Harris Chain of Lakes". These waters do empty into the St.John's, either at the Oklawaha River mouth or through Rodman Resourvoir and Lock and the Barge Canal. Until recently the Rodman Locks have been closed because of a Manatee thing, but now have an opening schedule. I was really excited about the possiblities for boating until I checked with locals here.
Rodman Resourvoir was established for fishing, not boating. It has an underwater forest that is best suited for boats under 25 ft. The Oklawaha is impassible because of fallen trees. Locals seem to think that your proposed route would be best done in a canoe or Jon Boat. If that.
It wasn't always that way. The route used to be run by steamships. There is a huge battle waging about this waterway, some want to drain Rodman Resouvoir and some want to open up the Oklawaha again.
The best source I've been able to find about this waterway is a fishing guide map:
http://nauticalcharts.com/fm-Floridat.htmI haven't seen this guide, but would like to. If you have better or additional information on this subject, please let me know.
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Tucker Traveler38495.2079050926