No, there is no coroplast between the frame rail and the bracket.
I drilled the holes and let the bolts hold the bracket in place then traced around the bracket with a razor blade. Then removed the bolts and brackets and pulled the cut coroplast out of the way then re-installed the brackets and bolts.
Here ia a pic of the front on both sides where I had to cut/notch out the coroplast. Plus a pic of the front center.
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2011 Avalanche 340TG
2001 Silverado 2500HD Crew Cab
B&W Companion Fifth Wheel Hitch
Had
2008 Hornet Retreat 38BHDS (Good camper)
2006 Terry 320DBHS (POS)