In a word 'condensation' .
It is amazing how much us humans exhale 'condensation'
during our sleep.
It will condense on the cold surfaces and being that the slide wall is not exposed to any 'air movement' (trapped by the mattress and sheet) it would collect there.
If possible place something between the mattress and wall to hold it away from that cold surface....something like 2 shallow plastic containers ...not cardboard.
Only need a couple of inches to allow for air movement.
Also... crack a vent ...this will help also.
Hope this helps
'07 WildCat 30 RLBS ~ West Coast Edition ~
Trail Air - Tri Glide Pin Box
2000 F350 DRW 7.3 Diesel Lariat 89k miles
Edge programmer, 4" SS exhaust turbo/back
Vancouver Island, B.C., Canada