Richard & Cheryl,
The chemicals that were used in the mfg of toilet paper that were harmful to septic systems have been banished. The big problems were hydrachloric acid and formaldhyde. It is virtually impossible to buy a paper that contains any chemicals harmful to septic systems.
How well will the TP break down easily? There is the jar of water test but the best test I have found is to use the Kirkland brand until the tank is half full while driving around the country for a few days and then empty and flush. I use a sewer solution system and can empty thru the 1" hose most of the time just using gravity. Some times the effluents need the jet of water to reduce them in size but I've never had a problem with TP.
Lately we have been using another brand, don't remember what, but it is because the Kirkland rolls are too big for our loo. They don't fit well on the roller and the take up a lot of space.