I just went through modifying the wiring on our 5th. I was getting less than 9 volts at the axles. This is a result of using 30-35 feet of 12 ga supply wire. When the total magnet resistance in parallel is only around 1 ohm, a small resistance in the main feed wires(which are in series to the magnets) has a significant effect. The resistance of the wires that drop to the axles is less significant while the wires that cross the axle are least important to getting peak performance.
Based on the work I did, your 10 volts at the axles seems reasonable. I could only get 10 volts after a few mods. The only way you can do better is to run 10 or 8 ga wire the whole way back, along with heavier wire in the tow vehicle and a heavier 7-pin harness on the trailer.
