Interesting problem, you’re so close. I went on a Jayco tour and saw a model like mine being built; I watched and asked a lot of questions.
My guess is?
The ground wire going up, goes to one upper light and is daisy chained from there. I can only assume that the wire isn’t broken and it is the first connection. On my Jayco it is accessible (I think yours might be too). I might suggest that you remove one of the upper lights so that you can get at the wire and hook up a temporary good ground, this could confirm that all the grounds to the upper light circuits are good. This might give you another choice to ground the lights from another light if the one in the back is not accessible?
Good Luck
Added: a wire tracer is great idea,