Our French holiday home has a septic tank. The most important thing to do is check that the septic tank in this property meets all current legislative requirements governing septic tanks and is large enough to cope with the effluent from the property it serves when that property is fully occupied. Possibly an inspection and report from a suitably qualified engineer might be a good idea.
We had our septic tank replaced about 10 years ago because it was insufficient in size and had a new one put in that met every legislative requirement. In France it has to be inspected and given a certificate of approval every 5-10 years. The body doing this has the wonderful acronym SPANC
As far as using it goes, we use it much as we would when on mains drainage as at home in the UK. Usual toilet paper, we use bleach, but I do check all cleaning materials I use just in case they are not recommended for septic tanks, but so far so good.
In the 10 years since the new system was installed, we have had no problems of any sort with ur system, despite pretty heavy use in holiday times for six weeks at a time.
We have been told the system needs pumping out about every 15 years. Longer, for us as it used far less than normal over a year.