delericho
Legend
Because of the solo nature of play, we have agreed that the PC (and crew) will simply "follow the left/right wall" and encounter what may...
Okay...
In short, the player essentally has an "I WIN" button in his PC/NPC group of and will flat out refuse to continue the adventure unless he has assurances that ALL treasure has been found.
So, as a DM, what would you do in my situation?
Because the player is clearly not interested in dungeon exploration as such, and just wants to purge the level of all monsters and collect all the loot, I would recommend redrawing the maps for the remaining levels to make them entirely linear in nature - one room, followed by the next, then the next, then the next, and so on. There are no other trails, so once the PC gets to the end, and finds the stairs for the next level, he can be assured that he's found everything.
(Normally, I wouldn't recommend this, as it is basically railroading, but given the first quoted paragraph above, I don't think it actually matters in this case.)
In future, for a true megadungeon, you might take a leaf out of "Expedition to the Ruins of Greyhawk" and work without a map at all. Handle dungeon exploration as a series of Skill Challenges, with success leading the party to one of your prepared 'easy' challenges and failure leading to one of your prepared 'hard' challenges (where the XP budget for the hard challenge is increased by the same XP value the party would have got had they succeeded on the Skill Challenge. Either way, the loot should be the same).
With this model, you don't even need to have especially defined dungeon levels - as the PCs proceed deeper into the dungeon (and gain levels) they naturally encounter tougher opponents. Just narrate them going down, up or across regions as you see fit.
(I also don't recommend this model for 'normal' dungeon investigations. However, it does seem to fit with megadungeons, and especially a megadungeon such as Moria where the majority of the complex is fairly devoid of life, and so not of too much interest.)