Have you had a chance to look into it (in the literal sense)?
Some are formal tanks; some are older style, designed to filter and then leak the excess liquid out into the surrounding soil. Size also varies a lot. As a result, everyone's experience may be different, and much of it not relevant to your tank.
You won't know which it is without looking, or seeing pictures, unfortunately. If it's the former type, then you'd need it emptying every now and then, and the local water company usually arranges this (often on a kind of subscription basis - once, twice, four times per year)
If it's the latter, it's probably a case of checking it every now and then, but leaving well alone unless it appears very full.
As an example, ours (from the 80s?) is made of standard red breeze-type blocks (tijolo furado), and it's large (you could stand up in it, and probably lie across it - though I won't be testing this out!). Every few rows, some of the blocks are at 90 degrees, so that they allow liquid out. It doesn't seem to smell, fill up much, or indeed have much evidence of paper etc.
I would suggest you only put toilet paper, and products marked as suitable for septic tanks, down the drains - this can be hard to remember if you've been cleaning with bleach, say, where some people would tip the bucket down the loo...