It sounds like somewhere there is excessive resistance, causing a voltage drop. Measure the voltage starting at the start battery under the hood, then follow the circuit (cable) and continue checking at every possible point. It's best to use the same ground point when checking too, that's to say, use a long jumper wire from the under hood battery to measure along the circuit since grounding to the frame can be problematic. When you see the lower voltage, you know the drop is forward of that point. Also check your ground connections, I discovered one battery ground on my 7.3 was corroded at the frame, and since it was bolted through a large hole in the frame, the connector had a minimal amount of contact. Lastly, the steel frame is a relatively crappy conductor compared to copper, so running a dedicated ground conductor will help.
Agile Ride improvement package