Lack of any caulking or gaskets where the bumper storage lid meets the body of the trailer is a common cause of this problem. If you search the forums for "sealing rear compartment" or "rear compartment leak" it should turn up what has been done to fix this. I have not had this problem withi my 2001 (knock wood) but it generally involves removing the molding and its chrome insert, and the bumper top and caulking the area.
"You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO
2002 Classic 30 Slideout
2001 Safari 25 RB Twin (Gone, but not forgotten)
2013 Dodge 2500 Laramie 4x4 Megacab Cummins
WBCCI 1322, TAC FL-39, AIR 82265