Normally when you smell sewer gas it is due to either a partially blocked drain or evaporation of trap seals. Try checking the tank water level to make sure your getting a full flush.
You could also contact someone to run a garden hose/water jet down your roof terminal (top of roof vent pipe) to make sure it is fully functional.
Try also not only flushing the toilets BUT the shower/bathtub basin drain to make sure the main sewer is completely flushed out.
Look for inside floor drains as the trap seal may be compromised from capillary attraction from mop string lint etc.
You can also try throwing Clorox down the worst smelling toilet to kill some bacteria. Good luck