I started a quest to help a sim find some stuff in a Tomb. Problem I am not sure where it is. I went to the Valley of Ancestors but I don't think that is the right place, there is nothing at the Dragon's Maw. Really lost here!

It depends on which tomb you're looking for. Of the one's I've found there's a step on trigger located in front of the market area which reveals a hidden tomb, there's also a hole in the ground behind the market right along the back wall which will reveal a tomb. There's another one to the west of the market (using the market as due north) inside the building with the four statues in front of it. It's #15 on this map

Also if you go into map view sometimes it'll mark where you need to go and there's a box that pops up about the UI that sometimes you can click on to automatically navigate where you need to be.