Locating the source of an odor can be a very difficult task, however I will discuss some of the more common sources in hopes that it may help you narrow down the search.
Obiously the most common source of sewer gas smell is a leak in the DWV (Drain, Waste & Vent system.) If the bathroom is built over a crawlspace or on slab construction even a sizeable leak may go unnoticed for an extended period of time. If it is over a crawl space begin by looking in the crawl space for evidence of a leak. Usually if there is a leak in the crawlspace the odor will be very evident in there. Also examine the piping for evidence of water, or stains streaking down the pipes that would indicate there may be a minor leak. Examine the floor of the crawlspace for any signs of standing water or in a worst case, signs of wet toilet tissue clinging on pipes, structural materials or on the floor.
If the bathroom is constructed over a basement examine the entire floor area from underneath. Even minor evidence of water may indicate a leak in the piping.
Is there possibly an unused drain in the room? A floor drain, seldom used shower stall or perhaps an unused laundry standpipe? If so, the water in that trap may have evaporated out. The solution is to pour about a quart of water into that drain to refill the trap, then add about a 1/4 cup of liquid cooking oil, which will float on top of the water and retard the rate of evaporation, while still leaving the drain fully operational.
Check in the cabinet under the lavatory bowl. Is there perhaps an air inlet valve (studor vent, or cheater vent) that is not functioning correctly? It will look like a vertical stub of 1.5in diameter pipe with a black plastic cap on the top with slots on the sides of the cap.
If you have a service access panel for the bathtub, open the panel and examine the subfloor area under the tub for signs of a leak. (You may need to use a flashlight and mirror to examine that area.)
Depending upon which plumbing code is in effect in your area, there are some other areas to check. The Uniform Plumbing Code (UPC) requires all vents to terminate through the roof to open air, However, the International Residential Code(IRC) only requires the main vent to terminate through the roof. Although never a good practice, the IRC permits auxillary vents to terminate in the attic space. If you have a vent terminated in the attic space you potentially will have two problems. The sewer gases will collect in the attic space, and given tht sewer gases are heavier than air, they will then permeat down through ceiling and structural materials into the house. The second problem is that excess moisture in the form of steam from how water discharge will condensate into the insulation, which will not only cause odor problems but can render the insulation useless.
The IRC also permits horizontal vents which terminate through the wall in the same manner as a laundry drier vent, subject to some restrictions about the proximity of windows, doors or other openings into the structure.
Sidewall vents are required to have screens to prevent birds or rodents from entering the vents, but screens will corode away rather quickly and require periodic inspection to insure the screens are in good repair and the vents are not obstructed by bird or insect nests.
You stated that the water level in your septic tank was up to the level of the drain input line. That is typical and is a sign that the tank is working correctly. Basically a septic tank is a large square concrete box that has an internal baffel. The waste enters the tank from one end and is contained by the baffle while bacteria break down the solids. Water then discharges from the opposite end of the tank and goes into the perforated leach field pipe where it is absorbed into the soil. Since water seeks its own level, it can pass through the baffle while maintaining the same level in both compartments.
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