Good ideas and thoughts all ... more things to check.
I didn't have long enough meter leads or an extension to cross between the fixtures to use the ground on the good one, but will tonight.
Same behavior with slide in, out and spots in-between - I examined the wire bundle under the slide where I could see it, and there are no pinches, kinks, twists, abrasions or cuts anywhere visible. Tonight I will probably drop the coroplast belly and chase the wires further.
The voltage drop regardless of the size of the load really doesn't make sense - Ohms Law says it shouldn't happen that way.
I did eliminate the switch by unplugging and jumping the wires at the switch. No change in behavior.
Furnace and fridge operate normally. Just got home from our first outing of the season and no issues with either.
I did examine, remove and re-seat the wires in the lugs on the DC distribution board. All are clean, no loose strands, stripped properly and lugs were snug.
I may have to pull the fuse panel out to find the grounds. They're apparently hidden somewhere behind the panel (or hiding in plain sight over on the A/C half of the panel ... but I didn't see them last night.
So, I really do have 2 separate issues:
First, the low voltage to light fixture (light 1) with any load, properly on Circuit 15, orange/white wire all the way from fuse panel to switch to light. Perhaps a bad light fixture? I didn't cut away the crimp wire nut and completely remove the fixture, just probed into the back of the wire nuts.
Second, (multi-part) the normal light fixture (light 2) "should" be on Circuit 15 (just because my OCD says it's in the slide out and should be on the slide out circuit), and they wouldn't split the circuit for load, as (2) 3 bulb fixtures would not overload a 15 amp circuit. No reason to split those into two circuits.
Then, the oddity of light 2 connected to red/white wire, and red/white wire at the panel is obviously not continuous to the fixture, and apparently red/white fixture segment is fed by the yellow/white from somewhere downstream of the panel.