Hi Paul. My RIB has been kept outside with a good cover on for the ten years I've had it. So far no problems with the tubes or anything else.
I usually leave the tubes as they were when I hauled out for the winter. They do go very soft, even sag a little, because of the low temperature.
If I have to climb aboard regularly - like if I'm doing some work aboard - I pump them a bit to stop the deflection as I climb on. A sunny day, even in midwinter, makes a big difference in tube pressure, so I never put very much air in. Like I said, no problems so far.