I just ordered a Valley Fifth Wheel in bed 7 pin extension for Dodge RAM with trailer wiring from etrailer.com (free Xmas S&H) for a little over $50. Based on instructions/illustrations, it plugs into existing wiring harness plugs in parallel /"in between".
Online trailer wiring searches yielded wiring color coding, etc and eventually pointed me to this easy/foolproof (hopefully) way to supplement the bumper plug. Need to get a hole saw to drill a hole in the bed wall, that SHOULD be the hardest part (?).