Viv,
Allow your mechanic to post using your site access because for us to explain the potential causes requires some diagnostics. I would need to know if the entire bus is all the way down or if the issue is related solely to the front, LH side or RH side.
I cannot think of a single thing that would cause the simultaneous lowering of the entire coach.
Air system pressures as displayed by your dash gauges are brake system pressures only and the fact they read 120 PSI tells me strongly you do not have a suspension system leak, but more likely an issue with the leveling system controls.
I am leaving on a trip tomorrow and will not be assured of computer access but there are folks that monitor this site that can talk your mechanic through the diagnosis.