I do not think you will gain anything James, if you are looking for more gain, add a director out the front....what do you have the spacing at right now from the hot to the back?
Grounded in the back, correct?
I have played with this a little and it seems that if you ground the back, and set the hot antenna @ 72" in front of that it was showing a fair amount of gain out the front.....at least that is what I remember on my truck, but it was years ago.
I was using sticks, on a 76 dodge truck ( long bed) with a metal camper shell......are you using 10K`s?
The guys running 3 or 4 antennas seem to like to put the back about 86" from the Hot.
and leave your self about a foot behind that back antenna, it seemed to tune better that way.
You really need a field strength meter.
Did you ever get any pictures of the grounding you did in the camper shell?
73
Jeff