We have an old house with a septic system less than 10 years old. Sewer gas is leaking into our house after showers and laundry. It is so bad we have to open windows, turn on fans and burn candles. It seems as though it is coming through our basement wall however plumbers have looked at our pipes with a camera and not seen any cracks, we have had the smoke treatment finding no leaking joints. We had the basement wall sealed with new piping. We have a sump pump in the basement. We have spent almost $5000 with no results. What could this be?

My first thought is a floor drain. Some have a back flow check valve that isn't working properly; or a clean-out cap that is missing or broken. It can be very difficult to trace old pipe in a house that has been extensively remodeled over the years, but this could lead you to an answer too. Not properly abandoned sewer pipe, or bad plugs could cause smell as well. I found a broke-off soil pipe under my kitchen floor (2nd story) that leaked sewer gas for 70 years! Its at the end on the left. Notice the patina!

You mentioned a sump pump. That is not hooked to the sewer line-correct?
When was the septic tank last pumped out? Where was the smoke introduced into your system for the smoke test?