About every 4-6 weeks, the shower and tub drains emit sewer odors into my home, and also when water from the washing machine into the sump or well. Pouring bleach in the drains kills the smell, but what could the problem be and what is the long-term fix?

Showers and drains allow sewer gas inside when they don't have enough water in them to make a seal. Evaporation could cause this but that takes several weeks/months of disuse. They could also lose water to siphoning caused by a blocked vent stack. Vent stacks are best rodded from the roof if its safe to be up there. Run a rod as tall as the house plus another 10 '.