No 12V at the coil while cranking is one of 2 things.
Either a bad control board or a broken wire between the control board and the coil.
Remove the control panel. The control board is attached to the inside of the control panel. There is a 12-pin connector on the panel, with all wires numbered. All of the wires will begin with J1- and then the nubmers 1-12.
Set your meter to ohms and conenct the black lead to a good ground. Leave the plug conencted to the board for all of the following tests.
1. Check for continuity between wire J1-1 to ground. This is the ground wire and should show continuity.
2. Set your meter for VDC and check for voltage at pin J1-7. It is the control power for the board and should show 12V.
3. Check for voltage at pin J1-12 while cranking. This is the output to the coil. It should show 12V while cranking.
If there is no continuity at pin J1-1, either that wire is broken or disconnected from the lower terminal block located all the way to the back of the control box.
If there is no power at pin J1-7, either that wire is broken or is disconnected from the 'I' terminal of the start solenoid.
If the first 2 are good but there is no power to pin J1-12 while cranking, the board is bad.
If there is power at J1-12 while cranking, then that wire is broken between the plug and the coil.
Also, that 10A fuse is supposed to be a 5A fuse.
Please let me know what you find.
If we discover the board is bad, you will have 3 choices.
1. Get a new one from Cummins.
2. Get a new aftermarket one from another company that makes a good quality product for much less money.
3. Send your board in to that company to have it rebuilt for even less. They offer rebuilding services for about $75 for most Onan boards.