I have a undercover flex on my new denali. I also wanted to go away from a vinyl one. the flex is a hard shell, tri fold. folded in thirds, the front section lays flat, the other two on top and then buckles to a latch. they claim in this position it will clear a fifth wheel hitch.
FWIW I love this unit. more security than the vinyl ones, looks better, stout enough to lay things on or stand on, pretty much waterproof (I live western oregon, we get rain). Occasionally it will get a few drips of water inside.
Course you pay for the hard folding kind vs. vinyl. but IMHO worth it.
If you want the whole bed exposed, it folds upright with two latch arms. Done that when I haul my motorycle, and works well, but you give up visibility through the rear window.
When I got mine, the dealer was also selling a Backflip IIRC, but they quite selling it since they were having more complaints with it vs. the Undercover flex.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!