Hi Paula,
I'm not an expert on this subject but for what it's worth, here is my experience with septic tanks.
When I moved into my property just over 10 years ago, the tank hadn't been in use for a couple of years.  There was no evidence that it had "dried out" and it functioned as expected.
Both our toilets (only one used 99.9% of the time), the shower and kitchen sink drain into it.  There are two people living in the house and maybe there is an ideal relationship between the size of the tank and the volume of waste water that discharges into it.  I only use a couple of the locally obtainable sachets very occasionally if I detect any trace of odor. I don't think it's something you need to be too concerned about.

You can restart the septic tank process by using 2 x nbs sprang sachets and then one every month. The tank shouldn't be allowed to dry out though.