Ahoy, mateys! Ye ready to board me ship and join me on the quest for the treasure of Captain Flint? Ye had better be, because ye'll be facin' some of the biggest challenges of yer life! Flint may have returned alive, but he told many tales of ferocious beasts, harsh weather, and mutiny. He gave me this map just before he died about a week ago. Are ye ready for the swashbuckling adventure of a lifetime, ye scurvy scalawags? Ye better be! Yo ho ho ho!
Here's Captain Flint's treasure map:

I saw this on the back of his map. I doesn't know what it means, be we be needin' all the help we can get, matey!
II,XXI XX,XV XII,II XVI,XX XVIII,XXVI XVII,I XXIII,V XVIII,XXVI XVII,I XVI,XVII XVI,VIII I,XV III,XII V,III V,XII XXI,XVII V,XXIII,XIX XXIII,I XVIII,I XXIII,V III,I XV,XIX XII,XVI V,XXIII I,XI XXII,XVI I,XXIII V,XVII XVI,VIII V,XVIII I,III V,XXIII V,III XVI,II XIII,XVI I,XIII III,XIII XV
Once you've solved it, please don't give out the coordinates. Let others have the experience of working the puzzle out themselves. I always take hint requests via email after FTF on any of my puzzle caches.
Note: Checker only responds to coordinates for Stage I

You can validate your puzzle solution with certitude.