<... The alternator is 100A ....>
It makes sense that there might be a "spike" of output from time to time (particularly just as the unit comes on line).  Since the alternator B+ cable for that unit should be awg 4 or 6 (minimum) and the purpose of the fuze is protecting the adjacent wires in the harness from collateral damage if the B+ shorts and tries to melt down, a 130-140 A fuze wouldn't be the least unreasonable.

<... My leading theory is fatigue ...>
My guess is that if the fuze near the break is all smoky looking it's not fatigue :o)

<...  fuse is flimsy ....>
I'm trying to visualize a dainty, flimsy 100 amp fuze.  Help me out here :o)
I'm going with a fusible link or more likely, a huge spade/blade fuze (like the smaller ones in the late model cars) for my application.  It will be no more than 6" or 8" from the alternator.

My best guess ... Jim S.
 

Russell Duffy wrote:

  The alternator is 100A, and I originally wondered if a 100A fuse would be enough.  As you mentioned, I had a larger fuse than the alternator rating on my RV-8.  ...  My leading theory is fatigue. ... The fuse is flimsy, and I have a bracket built to keep it from flexing.