Lisa H., call the plumber back, and have him perform the necessary tests to determine if in fact the odors that you are smelling are in fact coming from your new septic system. The odors may be caused by a natural gas leak, or from some other source.
The first test to be performed is a peppermint oil test. If the result of this test verify that in fact it is sewer gas, then look for a vent system leak. If the leak cannot be located, then have the plumber perform a smoke test so that you can visually locate the vent system leak. JWA