Last year I replaced my original air springs with some Arnott springs because they would drop all the way down when my Disco was parked outside overnight in below zero temps. It looks like the Arnott units are not much better. I took this picture this morning when it was minus 2 degrees fahrenheit.

 

Take a look ar RAVE rear suspension section. They shouldn't lose pressure that much but you can hear them 'settle' sometimes once you shut down the engine. It could be a faulty NRV in the compressor circuit (guessing) or it might need a compressor refurb. The air circuits are very susceptible to dirt and dust. Have you cleaned the air intake filter before, it's behind the rear light on the drivers side.

Does the SLS recover to normal when you start the engine? Do you have ACE fitted? Have you tried parking facing the other way, facing downhill?

 

The was the first time it's done it this year. It was also the first below zero weather we've had. It comes up as soon as it's started and will stay up through the night as long as it's above 10 degrees or so. Could be the fittings just bleed off if the temp is really cold. We're due for a warm-up on new years day, so I'm really not worried about it. I cleaned the filter a few years ago, I might take a look at it after I get it inside and warmed up next week.

 

Depending on the ambient when the bags are filled, that might just be physics (thermodynamics, specifically) having it's way with you. It's the same reason why you have to add air to your tires as the winter presses on.

The other cause could be a bad SLS valve block.

 

You could be very much right here. If the daytime use temperature was significantly higher than the nightime standing time the air would 'contract'. Also the compressor air during the day would be 'warm' by comparison to the standing night air.

 

TIP: with SLS air suspension to conserve the life and reduce the possible bag damage when jet washing under the truck raise the suspension and clean the air bags of grit and debris. Don't direct a full force jet onto the bags but use a diffused fan spray otherwise you'll 'drill' the bag and damage it.