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If it's a grease trap, I can't imagine one house producing that much
grease so as to be a problem. If it's some sort of of back-up valve,
the sediment has filled up to the pipes, and so I don't see how it
would accept much backed-up water before sending it on into the house
anyways... however... in case it still has some value as an access
point for roto-cabling further down the line, or as a water-back-up
tank... I might simply take out the tank, and instal a "clean-out" tap
a few feet closer to the house so as to avoid paving over it... the
only question is, I don't know if a clean-out access tap can have a cap
that allows back-up to flow out (sort of like a pressure release
valve), and thus avoid backing up into the house.

Though to be honest, I wish I knew exactly what the thing was.

In a 1961 (copyrighted date) version of Popular Mechanics Home Handyman book
series, there is a picture of what you describe. Made out of vitrified clay
or concrete. It is described as a grease trap and it says it was usually
only a necessity on farms where a large amount of meat was processed. I
assume they mean the farm processed its own pork and beef, etc instead of
buying meat already butchered and cut up from the market. It also says that
some states and localities also required them. It further says you should
not have one of these if you have a garbage disposer but should enlarge the
septic tank by 50% instead. It doesn't say anything about maintenance but I
would assume the farmer opened the top and cleaned it out regularly. It
shows it as being 36" deep and 24" across with the house line coming into it
12" from the top and the outlet line designed so it picks up the outgoing
liquid about 6" or so from the bottom and then up and out 12" from the top.
It calls for a 2" min. line and altho it doesn't actually show where in the
sewage line, it goes, I can't help but believe it is designed to be in a
separate line from the kichen and then meets up with the bathroom lines that
would transport solids, somewhere outside, before ending up in the septic
tank. Its design doesn't look like it would allow solids to pass through
it. Of course it may have been a design used before the advent of indoor
toilets.

Tom G