I don’t think they’re underground fuel storage either. But it has nothing to do with the backfill, and what you say about it is incorrect as far as my experience goes.
I’ve spent much of the last year developing plans for the replacement of large underground fuel storage tanks (both for emergency power generators and fueling facilities) and we only use pea gravel for backfill on the plans we’ve produced. Similar plan sets I’ve seen also use pea gravel. None of the plans I’ve seen or guidance I’ve consulted or pictures on the internet of such things have used anything but that type of material.
Furthermore, the liner we use is permeable and only serves to keep the pea gravel from migrating into the surrounding soil. It’s intended to let ground water drain away. Nothing outside the tank (and piping) is intended to prevent liquid (water or leaked fuel) from draining away. You don’t want ground water accumulating around the tank trying to float it up out of the ground.
That said, I see no sumps/ports or anything along the top like a fuel storage tank would have and none of the piping.
Also, they would be way too big for fuel storage for an 8 unit condo, if such a small residential building would even have emergency power generators. These tanks are about the size of what we’re replacing for a large office building for hundreds of people, and they supply fuel to half a dozen generators, each the size of a small bedroom. (They would also be the right kind of size for a gas station.)
The picture at the top of this page shows a couple of fuel tanks before they have been completely backfilled with gravel. This is before all the stuff on top is installed, but the access ports on top for that are visible, unlike the pic in the OP.
http://www.tercenter.org/pages/usts.cfm
The picture at the top left of this page shows some tanks that have been backfilled with gravel and the sumps, piping, etc. have been installed. (This one is for a fueling facility as opposed to storage for generators but they look much the same.)
http://kesslertank.com/services.html
I don’t know what the tanks in the OP are, but they don’t look like fuel storage to me. If I had to guess, I’d go along with you and those above suggesting underground storm water detention.