Our old Teton had that issue in the outside corners of the bed we stuffed foam in there and put folded fleece over that ,all from the inside. That worked for us. Hope that helps.
My wife keeps saying I don't listen to her -Or something like that?...."My advice has no basis ,more reliable them my own meandering experience" Baz Luhrmann
2011 Sierra 355QBQ 1986 F350