wet locker cuboards ?

I have a auto trail chieftan se tag axle van,on unloading the van last week we have found mold damp ,in the cuboard above the rear wheel arches,so much so the upright cuboard is rotten and i have put a finger throught it,has anybody else had a autotrail chieftan on a 55 plate had this problem,if so what is the fix,i am not sure but has the whole bottom fibreglass skirt go to come off for access and refit,any help out there ?