I'll admit that I never played nor owned
Night Below (my loss, no doubt), but this sounds far and away more like
Dragon Mountain to me.
It has a dragon in a dungeon at the end of it, sure.
The Night Below comparison is only the first book - it was a sandboxy rural village county called Haranshire with loads of little sub-quests going on. Then the second two books went into the underdark, which is where TO SLAY A DRAGON diverges.
Dragon Mountain is a lot more "mega-dungeony" than TO SLAY A DRAGON. The dungeon is the focus of
Dragon Mountain, whereas iTO SLAY A DRAGON focuses equally on each of the three parts: the sandbox village and the start, the wilderness journey, and then the dungeon.
Dragon Mountain, as I recall, has a "gather some clues and find the mountain" section at the beginning.
So, yeah, it has things in common with both. It starts more like
Night Below, and ends more like
Dragon Mountain, and has more emphasis on the middle bit than either.