Mysterious camper wiring issue

Here is the issue.

The brake lights, running lights and turn signals work great... 

EXCEPT when the running lights or headlights are turned on in the truck. 

If the truck lights are on, then the camper running lights work, but the brake lights and turn signals do not.

This happens not just when connected to our truck, but when we connect to other trucks too. (Thanks for bringing your truck over today, Rob).

We've scoured the interwebs, and the advice is to:

1) check all the grounds (done)

2) check for a weak ground by adding a jumper wire from a good ground (on the frame)

 to each light assembly 

(that doesn't change anything, so a weak ground isn't the issue)

(but it did help us identify a weak ground in one of the camper running lights, which we fixed)

3) make sure there isn't an exposed or pinched wire anywhere in the camper.

Say what?

This is overwhelming.  David checked many places but how can you check them all? 

Has anyone experienced this? Or have a suggestion?

It's also worth mentioning that David hard wired a separate ground wire from the tail light assembly 

to the trailer frame. Those suckers are grounded.

This was all ok in summer when the days start at 5 am and it's light until after 10 pm.  But the days are getting shorter every day people! There will be a time soon when we need to pull the camper somewhere in the dark, with the truck headlights on.

Suggestions are welcome. Thanks.

Here's a relaxing photo of the last place we took the toaster camping. A cedar forest on the Kootenai National Forest near the Montana/Idaho border.  

UPDATE SEPT 11, 2013

Thank goodness for the Tin Can Tourist group on Facebook! People had several suggestions, which David tried, and now the problem is fixed! Huzzah!

He wired the lights backwards, I believe. He reversed the wires and now it works.  The internet is just amazing. Thank you friends! Now we can travel at night.  We are unstoppable.