Ok, so just clarifying the info we know about the Road Poneglyphs:
The first one was hidden in Zou's Whale Forest. Due to the Mokomo Dukedom's ties to Wano, it would seem the Great Kingdom gave them one, too.
The second one was hidden in Wano's Mt. Fuji. They had strong ties with the GK, as we know and also have Pluton, too.
The third one was originally in Fishman Island's Sea Forest, but has since mysteriously disappeared. Also, coincidentally, the Ryugu Kingdom has Poseidon, too and a second Poneglyph with Joy Boy's apology to them.
And then there's the 4th one, owned by Big Mom. Where she got it from is unknown, but judging by the two other nations having Ancient Weapons, it's likely either Elbaf or a Sky Island. And like Wano and Fishman Island, the possibility of Uranus being where the RP was, is quite possible.