Leaving house with septic system over winter

My summer home is at 8500 ft in the Colorado mountains. Although I drain the entire water & well sys and fill the pipes & drains with RV antifreeze each fall before leaving, I've never done anything to protect the septic sys and never had any issues in 6 yrs here. It gets down to -20 F here every winter. Some years we do have several feet of snow on the ground, other years its almost bare ground at least part of the winter. There are 100's of seasonal summer homes in this area, all on septic sys and I've never heard of any issues from freezing. Some neighbors who stay all winter do have issues with water lines freezing where they are aboveground in unheated areas or where they transition from above to below ground at their well.
None of us seasonal visitors leave any heat on during the winter when vacant for 6 months, it would be impractical, very expensive and a safety concern. I don't return to this home until late spring, when its above freezing.