We've been in this 30 year old house for 8 years, it sat empty for 5 years prior. Added a third adult to houshold 1 1/2 year ago, (increase sewage load) and now have sewer drain issues. We checked septic tank, seems okay, was perfect 6 years ago. Then snaked all drain lines, and found blockage in outside 4" line to tank. Cleared, or maybe not, and no problems this past evening. Flushed 2nd storey commode and after a minute or so had bubbling (not sucking) in commode. Found waterin floor tub, then had gurgling, (sucking) in first one sink and then another. Tub empty now, (1 hour) and all quiet. Your thoughts?

Have you been flushing wipes or other non degradeable items, they can build up a blockage, caustic soda can dissolve some blockages and should be used regularly in all drains. Issues don't come from additional load but can come from illness where antibiotics are used killing the anerobic activity and inappropriate material being flushed. If you have just had the pipes cleaned talk to your contractor because the problem may not have been where you thought it was. Do you have an aggitator, that can free blockages in basins, etc but a problem in an upper storey definitely think blockage

How long ago did you have the holding tank pumped?