Those two were LOOSE when the “last” diesel auto mechanic opened that access panel. He tightened them. And he taped the wires underneath, near the hole in the axle. And he connected the ground wire on one of the truck batteries (all mentioned in my original post). The issue resolved for 3 days. Then driving over aged highway macadam and it started again.
My brother mentioned the potential for the slightest bit of low voltage to mess with computer components in the truck.
Today I reopened the vanity access panel, looked around, took photos, buttoned it up again. Didn’t change anything. Noticed there are actually two junction boxes in there, street side one much harder to access; I opened the curbside junction box. Those two black ground wires still looked tight. Tomorrow morning I move to new location (more rural).
Confidence level is low.
Two mechanics looked at the ground bus bar under the dinette, after I pointed out where it is located, but I don’t know if anyone was certain about which wires on it were/are associated with the 7-pin cable and/or the brake wiring. And I shared the wiring diagrams (such as they are) in the manual. One tested the individual elements of the 7 pin head (trailer side) and said he didn’t think the issue was there, it was functioning.
Will do this in Cimarron, where my friend can help me.