Go to Ford's Truck Body Builders Advisory web site
(Click Here). Has the specs you are looking for concerning chassis 12VDC distribution but it will not show what Damon might have done tapping into what Ford provided.
For the electrical info you are looking for
Click Here and go to the last two pages.
Concerning the tach error, physically disconnect the chassis battery, and maybe the coach batteries also, for a few minutes to clear the chassis computers. When you reconnect you might see all the gages swing thru their total movement as the computers process thru their power on self test.