I will tell you my theory about it’s location and a possible background story for it.
After reading the whole theory you might get a good picture of the possible One Piece world, that we get to know anew over and over!
So let’s start with this!
What is the One Piece? – I don’t know but I most likely assume that its history and existence is tied to the Ancient Kingdom and the void century.
Since that we only know about one person who was able to complete the puzzle and find the One Piece!
Noone else than Gol D Roger!
But before I tell you how he got his way trought, I will throw in a question regarding the One Piece?
Why exactly noone was able, for certain centuries…to find the One Piece?
The road poneglypgs are especially hidden!
So in order to become pirate king you have to find the One Piece! – stated by Roger!
In order to find the One Piece you have to gather the 4 road poneglyphs!
So we can argue that the Yonkos want to become pirate king(or queen) in order to reach their accomplish their plans for the world. The only plan we don’t know about yet is Shanks’.
But since the years those pirates are the primary leaders in the New World, noone was able to posses all 4 road poneglyphs.
We know that Big Mom owns one, Kaido owns one and noone right now dares to clash for them to accomplish their goals!
Finally we know that the Mink tribe also protects a road poneglyph.
It is most likely that Rogers journey also started with climbin the elephant and befriending the Mink tribe.
All in all people made it this far as even copys of the road poneglyphs are rotating trough the world for expensive prices as we can guess!
Let’s imply this panel to it!
So how did the copies go into rotation at all?
I most likely believe that those copies around are copies from old owners and contests for the road poneglyphs. I mean between 800 years ago and now those road porneglyphs might have been contestet from some treasure hunters or even the world government.
That’s why I think those copies might exist in the world.
It is a foreshadowing to the future that copies might be important. I never would believe that Luffy will gather all 4 road poneglyphs and take them to Raftel… 
So this leads me to conclusion that a certain force is holding back the ability to gather all 4 road goneglyphs! It is noone else than…
The reason why pirates can’t become pirate king is simply because the world government is keeping the fourth road poneglyph.
Revealing the treasure to the world would reveals the journey to the world.
The world, all pirates would know that there is no way to become pirate king other than traveling to Mary Joa!
If we are correct, then even the road poneglyph on Zou was one of the hardest ever to obtain, I believe it is also a road poneglyph the world government doesn’t know about yet…
So finally this leads us to the question: How did Gol D Roger find the 4th road poneglyph at Mary Joa?
I predict that Dragon got the one and only copy of the road poneglyph of Mary Joa and passed it to Gol D Roger!
Let’s first have a look at the “Monkey D clan”.
Garp is the currently oldest known “Monkey D” he was a marine.
But he also wanted Luffy to become a marine!
I believe back when Luffy wasn’t born, back when Roger was a pirate, also Dragon was a marine of a high rank or even a Cipher Pol.
Anyway Dragon had a position that allowed him to guard or access the national treasure of Mary Joa!
All in all the “Monkey Ds” has been a family of marines, based on this theory.
But why Roger and Dragon?
I believe Dragon had a much clearer view of the history at that point.
Also a lot of people compared Dragon to Coby in the recent timeline.
Dragon might has been Rogers Coby and Coby in the timeline right now might revolutionate the world government in future!
But all in all I think Dragon and Roger somehow became closer and closer and so Dragon realized the truth of the world due to Rogers insights and so Dragon did a betrayal.
There is alot of stuff you can speculate about how this event exactly got triggered, but this just solves a big puzzle thrown in the fandom!
This is event was leading into Dragons retirement.
The world government most likely found out about this later when Roger became pirate king.
I believe Dragon disappeared and the world government knew about Dragon.
Dragon was hiding living a secret live and when roger got execute he decided to make a plan!
Years later he appeared from nothing and founded the revolutionary army!
I most likely believe that another goal of the revolutionary army is obtaining the road poneglyph of Mary Joa, wether it is for the army, his son or even for everyone!
That is what I think might be adressed also on the Reverie and afterwards!
Summary:
The fourth poneglyph is hidden in Mary Joa as national treasure!
That is the reason noone is able to find the One Piece!
Dragon back in Rogers era was a marine and had a position to guard or enter the treasure room of Mary Joa!
Gol D Roger and Monkey D Dragon had a closer connection due to the D and the true history of the world.
Dragon betrayed the world government and made a copy of the road poneglyph, which he gave to Gol D Roger.
Roger became pirate king and got executed.
In order to have another pirate king in the world of One Piece, Dragon created the revolutionary army and tries also to obtain the 4th road poneglyph on Mary Joa!
*Theory by Rej Discussions
