Electrical problems like you discribed are hard to find .... removing one circuit after another is one approach .... in other words, remove a fuse to make that circuit off line .....
Since you had a slow cranking as a first sign of the problem, indicating a poor connection in that electrical system, I would clear all electrical connection from and to the starter motor, ground wire to the frame, both terminals connectors on the battery, both posts etc..and connections on the starting motor .... the only connectors left are inside the starter motor, which at this time assume to be fine and the motor itself ......
As a double check, have the battery check again, Auto Zone does it for Free and checks the current draw from the starter motor which is very important ......

Bill
1981 300 TD