Well, I pulled out the tub surround.
camper wiring 8 by
gijoecam, on Flickr
The camper laughed at me, I swear it! (Maybe it was too much sun and heat?) I got mad.
So I pulled the tub too... Then I was able to pull the whole back panel out.
camper wiring 10 by
gijoecam, on Flickr
Here's the wiring for the rear left taillight, and the left side rear marker light. It's all 14 gauge wiring, so I'm 100% sure it can handle the load of all of them on one ground. Now the challenge was getting a ground connection point up at the marker lights!
I started at the upper left marker, and could tell that the wires came from towards the center. So I pulled the next one. When I pulled the center light out, there was a loop of green and white wire, but no terminals. When I pulled the right-center light, I found a pair of usable spade connections. Then I used a straightened coat-hanger to reach across from the left-center to the left-most light, and discovered there is a perfect wire chase all the way across the back of the camper. SCORE!!!
camper wiring 12 by
gijoecam, on Flickr
THe only thing left to do was to drill the hole to get down through the back wall of the camper. I popped the hole through at a 45 degree angle as far down as I could, and wouldn't you know it? NAILED IT!!!
camper wiring 13 by
gijoecam, on Flickr
Now I can fish the new ground across from the center, use the same splice connectors they do from the factory, run it down to the taillight, connect it there, and (hopefully) reinstall the tub.
Wish me luck!!