According to the plumber, the installation is all correct. The plumbing is all visible and accessible under the house. The shower drain smells of sewer before, during and after using the shower. I have measured the depth of the water in the p-trap at 4 inches. The vent is "teed" in the drain line about three to four feet past the p-trap. I have ran water from a garden hose down the vent on the roof. The sink and toilet vents all connect in the ceiling space before going through the roof. (one 2 in vent line)
The plumber opened a chase down stream of the shower and ran water from the bathroom and saw no signs of blockage.

How is the odor (substantial, not just a whiff) getting through the p-trap seal?