Charmin Toilet paper is safe to use in septic tanks.
I have a septic tank at home and have always used Charmin Brand Toilet paper without any issues.
Just don't use too much of the toilet paper as it can clog the septic system up and that goes for city sewer systems as well they can be clogged up and your toilet could clog up from too much toilet paper.
Also use some RidX in your Toilet for the septic system at least once per year to help break down the waste so that you don't have to pump your septic system as often.
It's recommended still to pump your septic tank out every 5 to 10 years depending on how often you use the septic system and how much waste goes down it.
Most all brands of toilet paper you buy today in stores are considered to be Septic Safe as well as city sewer safe.