I have the same shape truck, 2005 2500HD extended cab short bed, but Duramax diesel 2wd 5 speed allison auto. Logging about 100k miles on the highway I got about 22 mpg at 60 mph and then lost about 2 mpg for each 5 mph above that, so 20 at 65, 18 at 70, and 16ish at 75. I looked at the GM tonneau patent and decided that 44% was about the optimum and a convenient size so I could still reach into the bed with the cover on. Built it from plywood with a 2x4 frame so two big guys could sit on it in chairs. It is held down with ratchet straps and 1/4" line to the rear cargo hooks so it comes right out if needed, or I just lay it flat in the bed and put the cargo on top of it. It has never moved from wind, even on a short drive with the straps loose just to see. Here's a couple of pictures:
Did nothing below about 55 mph but added about 1.5 mpg at 60, 65, and 70 mph for sure, probably about the same at 75 and 80 but don't have good data. So basically it lets me drive almost 5 mph faster for the same mileage on the highway. It's been on for about a year and 15k miles so far with no issues. If you cut my plywood in half across the truck, added two more 2x4 cross pieces along the new edges, and then bolted them together with a couple of through bolts you would have the same thing for running empty but you could split it into two and lay each half out of the way down in the bed beside your 5th wheel hitch for towing. I bought some generic lug covers and pizza pans to make some wheel covers but never got around to that, and I was never ambitious enough to even consider your belly pan, but this was pretty simple.