Hi,
thanks for your advice during my "crisis".
I wanted to share with you how this worked out for your possible future recommendations.
It turns out that the Tub in fact had a P-trap and the venting was technically correct.
The problem was a sheared ABS vent pipe in the wall upstairs from where I thought it was.
The pipe is made by Centaur and there is a class action law suit against them and other companies that made faulty ABS pipes between 1984 and 1989 I think.
http://www.abspipes.com/
Apparently it was a huge problem out here in California, and it led some townships to ban ABS all together and drainage pipe can now only be cast iron in some places.
The interesting part is how to detect the hidden odor source. There are fuse lit smoke "candles" made just for this purpose. You can get them on the web or through a plumbing supply but I did have a difficult time finding a plumber who knew this technique. Anyone can do it, but be warned the heat can melt the ABS so do it in an outside cleanout or RV dump pipe.
http://www.metermall.com/product%20p...%20Candles.htm
Basically, you duct tape one of these candles to a wire coat hanger bent up to hang it inside on the lip of a clean out pipe to the tank or to the house to make sure it doesn't slip all the way down into the tank.
You light fuse, drop it down the pipe, hang it on the edge with the coat hanger, cover the pipe with a towel and use a small vacuum cleaner with the hose attached to the blower end and you blow the smoke into the system. About 2 seconds later, smoke is billowing from your vents on the roof, and if you have a leak in the house, out of the leak too. Worked like a charm, but in some cases (large houses etc), you may have to plug the vents on the roof for maximum effect. Not a problem in my case since the vent pipe was completely sheared.
Anyway, I hope someone else can benefit from my experience.
Thanks again for you help.