Rapid Voltage Loss while driving/idling

Suspect the chassis battery, even though it is new. Disconnect it from the circuit by removing one of the terminals, then check the voltage. Or better yet, take it to WalMart or an Auto-Zone/Advance Auto type place and let them load test the battery. While you are in there, check the terminals for corrosion or bad connection. Also check for a bad ground to the chassis (not likely the cause).

After that suspect the alternator diodes. The same place that checks the battery can check the alternator output.