Had the same issue on mine, but it's a pretty easy fix. If you crawl under it, you'll find that the DEF fill hose is routed over the top of other cables/hoses on top of the frame. Take the hose quick release loose from the DEF tank and reroute the hose under the other cables/hoses, then reattach it. That little change gets the hose at least level, and it will fill. Also, take note to fill when the rig is parked somewhere that leans to the right, or if necessary, drop the jacks on the left to tilt it and it will fill easier. Should go 8-9K miles on a tank, so fill it full so you don't need to mess with it for awhile
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