With the head literally in the center of the rig, with the slideout next to it, it's gonna be hard to find a tarp that will fit without a lot of overhang. You living in it makes it harder to cover the roof (slide out) without it looking like a 3rd world refugee camp.
A 40' x 20' will cover the whole roof (slide included). If you are handy with some basic tools, you can fold up the extra so it's underneath(leaving some overhang on the sides), and make new grommet hold downs and have a seamless cover, while extending part of the tarp over the slideout. The main downside of this is you can't open the roof vents, and you'll need to remove the black & gray tank and fridge vent covers, make cuts in the tarp to fit these, seal, and re-install the covers. 3/8" line and rachet straps will keep it held down and fold free.
2004 F-350 King Ranch.