POD is that Louis IX sends scouts into Mansoura who discover the ambush there and inform him of it averting the disaster of OTL. He besieges it and destroys the enemy army. He army remains intact and later conquers Cairo thereby Egypt. Levant subsequently falls. He later launches the eight crusade but unlike OTL he and his army don't have dysentery. This leads to the conquest of Tunisia
I think you're confusing battles, Louis lost because Robert tried to assault Mansourah with just the Vanguard when it was obviously a trap. If you want them to win the battle just say that Robert doesn't assault Mansourah, Louis arrives and lays conventional siege. After that, victory is almost assured, the Ayyubids of Egypt are both internally and externally in their worst moment. So, Egypt is conquered, I would say that Robert of Artois will be the one to be crowned king of Egypt
The Levant won't fall to the Crusaders just because Egypt fell, the Ayyubid position in Syria is pretty strong. Louis will stay after the capture of Egypt and complete his pilgrimage, but he will probably just retake the places that are most strategically valuable and most significant for Christianity; so, Jerusalem, Nazareth, Nablus, Tiberias and Ascalon return to Crusader hands. He may also try to help Antioch as its situation at this moment was dire.
In the following years Robert will need to consolidate his position, which I am pretty sure he's capable of, dealing with the remnants of thr Mamluks, capturing the Red Sea ports and fighting against An-Nasir Yusuf, Sultan of Syria, along with the other Crusader States. Given his enemies, he will most probably win.
The Mongols will invade and destroy Aleppo (I bet An-Nasir would do exact same stupid thing of trying to get the Mongols to take Egypt for him and failing completely), the Crusaders have the option of fighting the Khan alone or siding with the Ayyubids, both are possibilities; they won't accept to pay tribute because of victory desease after the fall of Egypt.
The eighth Crusade won't be sent to Tunis, the reason Tunis was chosen for the eighth Crusade was the failure of the Crusade to Egypt, so the Crusade when it happens will be directed to the Levant, to the Mongols or the Ayyubids, it depends how the Crusaders reacted previously. In case the Mongols are the ones to be dealt with, it would be very interesting to have another crusade launched from Anatolia, as Michael Palaiologos has all reasons to give full support for this Crusade. Even if it's only sent to the Levant by sea, it also serves as an opportunity for Manfred of Sicily to repair relations with the Pope. So, the Greeks will be benefited from this Crusade in every case.
Louis IX will pass as the greatest king of Catholic Christendom to that point, conqueror of Jerusalem and Egypt. So, even more places will be named Saint Louis.