I'm not getting any voltage drop through 20' of 18 gauge wire. I may be doing it wrong.
The amount of voltage drop over that distance depends on the amount of current (amperage) going thru the wire.
No current means no voltage drop. If the Prop is at full power (white) and you still have no voltage drop, then that power injection may not have been needed.
Voltage drop is the difference between the voltage at the source and at the injection point when the Prop is lit.