[vladjjj]

Actually, a proper septic tank (according to EU regulations) has to be water-tight and be emptied regularly. Many beaches on the Mediterranean loose their health pass occasionally due to nearby overbuilding and improper septic tanks.

[jjeaff]

That's not a "proper" septic tank. That's just a holding tank. The word 'septic' alludes to the fact that you are promoting putrefaction to decompose the contents.

RVs have holding tanks that must be emptied. Thus, they aren't called "septic tanks".

But I'm sure the term is used loosely.

[jjeaff]

That's not the case in the US.

[TheMiller]

At least, it's not the normal case in the U.S. We call that a "holding tank", not a septic tank.