[tsubaki]

Umkay, much, many things to run through!
Check the batteries with a load tester.
Check terminal connections at batteries and solenoid.
Whack solenoid while someone else is attempting to start.
Whack starter while someone else is attempting to start.
(whacking, is done with a wooden handle)
Bypass wires from battery to starter solenoid, looking for broken main terminal wire (with jumper cables, be sure to do both positive and negetive).
Bypass solenoid (very briefly), to test starter and wire connection effectiveness.



Where are we at now?


If everything but the starter is engaging, it ain't no fuse.
Chances are it is a bad connection or bad starter or solenoid.