Yes Charmin Ultra Strong toilet paper is safe to use in septic systems as well as many other sewer systems.
Charmin toilet paper company tests their toilet papers to ensure that their toilet papers including the Charmin Ultra Strong toilet paper breaks down properly in municipal sewer systems as well as septic systems and anywhere else that the toilet paper may go too when flushed.
Any toilet paper that is septic safe should say on the package that it's sewer system and septic safe which means it breaks down good and fast in a septic system as well as a regular sewer system.