We have a cottage that at times has septic odors.The traps are always full . I have torn down walls looking for unglued or cracked vent joints . I have replaced all drain lines . A heated vent and a solar powered vent have also been installed .Any other suggestions??

Does your cottage have a crawl space and if so have you checked for leaks in sewer piping (possible underground). Can you associate the odors with any specific weather condition. Could you have rain water getting in the crawl space and stagnating?

Basement floor drains have traps and can evaporate dry. A bad wax ring on a toilet will also cause this issue. Have you checked the vent stack in your attic and on your roof?