You can keep any of the tonneau covers on while towing a 5th wheel except for the one piece hard covers.
I have a hard roll type cover (Pace Edwards Full Metal Jackrabbit) that I can roll all the way in, or I can roll it to just in front of the 5th wheel hitch, and still tow. With it just in front of the hitch it hides what ever may be in the bed in front of the hitch. And the 5er overhang hides what ever is behind the hitch. Locks every 12".
And I roll it all the way back to the tailgate when unhooked to hide things in the bed.