If the gap is deep, I see another mentions a filling rope sort of thing
to
bring the level up.

Where is the gap? At the base? Or along the top and into the wall? A
vertical oriented gap is a little different from a horizontal one but not
all that much so.


The home is old maybe 55 years old. so this is long term settling.
The gap I'm trying to fill is horizontal along the top of the tub and
bottom of the tile.

Ok, sounds like settling to me too then. Not too deep in? Caulk will work.

Why will the rope filler make it look bad?

I dont know. Unless the person was worried it would get wet and bulge out
or something? Me? I'd just use layers of caulk unless it was really deep.
If it was really deep, I'd look at the local hardware store for something
'plasticy' that they use with setting windows in to sort of make a 'shim'
that fit in there then caulk over that. From your description, it doesnt
sound like it would be deep enough to need that.
xxcarol