Re: 84 days
The Apollo capsule must have been powered down and designed to leak as little as possible. Skylab had its power from solar pannels.
The Shuttle sometimes uses the EDO, Extended Duration Orbiter, that enables it to stay up to 3 weeks. It's just more fuel cells.
But if the Shuttle were linked to an unmanned platform with huge solar arrays, then it can stay far longer but it's not practical as the Shuttle is meant to go back and forth to space.
Raoul