p30 cranks, wont start

[Kristamarie]

Neighbor (20's) has a 1990 Chevy P30 (350) Ice Cream truck.
They (w/friends) attempted to install an inverter, but in the process they lost control of the positive battery cable to the inverter and it shorted to the body.

Now the truck cranks, but won't start.
Checked the battery...good
Checked the alternator...good
Replaced the ignition switch

No accessories at all
8 volts to the fuse box at radio, etc.
12 volts at hazard and horn
Voltage changes (0 & 12) at ign fuse when we turn the key

Any suggestions would be helpful. I showed them how to use a multimeter, and will over see their work but I have no idea where to go from here and cannot find any wiring diagrams anywhere for this thing.

Anything is Appreciated,
Krista