Only someone with a 1999 Pace Arrow that has done this task can give you exact instructions. I have installed the dual rear cameras, and have posted a writeup on IRV2. The steps and pictures may give you a general idea of what you're facing.
To see the writeup, go to this link
files section of IRV2and scroll down until you see the topic "Rear View Camera (2) and monitor install".
If you're going to install 2 cameras, then don't waste time working with the old cable as it will not be sufficient to run 2 cameras. You'll be running cable down the back cap, under the motorhome, and back up into the cab area.
Fred
Fred & Vicki
St. Augustine, Florida