Thoughts on how to fix a depression in the roof

Just keep in mind that if there is a crease of any sort, the only proper fix will be a new skin. If its just a depression that is oil canning you can pop it out and provide some support from underneath and it should be fine.
I dont advise using filler at all on a roof except for around the edges. The roof skin expands and contracts with temp and eventually the filler will come loose.