OK, so thanks for the clarification between "neutral" and "negative". Understood the concept, not the phrasing :)
I redid all the suspect wiring over the weekend. Cut all the wires back beyond the corrosion and reconnected with split bolts, which I taped (plastic - rubber - plastic) separately and secured so they don't bounce around. Now it seems everything is working great. I'm still showing a small voltage drop (~0.25v) and I suspect I have a ground connection somewhere that is corroded or suspect. After running everything in the unit I could for a few hours, the wires were still stone cold, so I think that problem is solved.
Now on to figuring out why the step doesn't work now - thinking it may be related to my last little bit of voltage drop. But - I'll play with it and put a new post out if I can't figure it out.
Would there be any issues with removing the step entirely and working on it on my workbench instead of laying on the ground under the motorhome?