For about the past month we have noticed what appears to be a sewer gas odor in all areas of drains in our 2 1/2 bath home. Also notice a large "air bubble" in the toilet when it is flushed (as soon as it is flushed, before the water leaves the bowl, a large bubble of air comes up). Also the water level in that toilet is lower than what it used to be. When the upstairs toilet is flushed some very small air bubbles come up as the water is leaving the bowl. Both toilets "rock" very little.

The "bubble" indicates a blockage. The discharge hits a clog and bounces back sending a bubble of sewer gas ahead of it. Are you on city sewer or a septic tank? By, " Both toilets "rock" very little." do you mean they are both loose? Let us know. Tom

I'm on city sewer. This toilet (with the "bubble") rocks a little more than the other one, and I "think" it is the last drain before it goes to city sewer. Would a blockage cause other toilets, or, drains to "bubble"? What about the water level in the bowl, does that provide other indications?

This could be a local blockage in the toilet or clos in the toilet drain line. Have you used a closet auger,(see image) and augered out the bowl? Since the toilets rock It's a pretty safe bet that the wax seal's ruptured on both of them letting sewer gas enter your home. I would auger the bubbling one and then pull and reset each with a new wax seal. Good luck, Tom