How long since your radiator bank has been cleaned out? As Rhein said, the fins might be clogged with oily dirt, but critters also like to fill in between the units (you probably have 2 or 3 in a stack back there) with leaves and twigs. I had an issue like that a couple of years ago, took it into a Freightliner shop. Couldn't believe the pile of stuff they pulled out, and the gunk they washed out of the fins.
John & Diane, fulltiming since '12
'02 DS 40, FL, 3126, '04 Element
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element