Is the pan built on concrete slab or a plywooden subfloor? How thick is it in the area that sounds hollow? If plywood, did you staple down some lath?
You can fill the dips in with thinset. The type you choose will depend on how thick it needs to be. Normal thinset can be used up to about 1/4" thick. If it needs to be thicker, then, I'd use a medium bed mortar (often called granite and marble mortar) - it can support thicknesses up to at least 1/2" in one layer.
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