1971 Questions for all other cars

Electrical problem
1971 Questions for all other cars V8 Two Wheel Drive UNKNOW miles

coming home all of my electrical guages quit, fuel, oil pressure, voltage indicator, and water temp. Vehicle is a 1971 flxible bus (ex LA transit) I converted to motor home. Dash is group of individual instruments, not a cluster. Replaced idiot lights. Do not have the diagrams any more for wiring. Suspect I lost 12v or ground. Do not believe problem to be sending units as all went out at same time. Is this correct wiring? Sending unit to one terminal on guage and other side to 12v positive. Case acts as ground. I cannot find battery on any of the guages and dash lights do not work as well. Bus is 24v for bus engine and lighting but I took a 12v tap off of house batteries for dash and 12v accessories. Can I run a new positive from 12v buss bar to an instrument or is polarity going to be a problem with each gauge? Need help ASAP, trying to leave on trip 1 july.