I have a 1999 Elddis twin axle which has some damp ?

My 1999 Elddis twin axle had a bump to the nearside roof some time before we had it and now that side shows signs of some damp. There is a particulary soft are about 6inches by 10inches at the bottom right hand side of the small front window (noticed because the blind framework is coming away. Its as if there is a hollow gap behind the walling. Can I do anything about this such as inject some kind of filler into it?