Most people prefer to use a mattress pad. The seats make for kind of an irregular bed, and the pad evens out the peaks and valleys. On the EXT, the bed is a queen size. On the non-extended (or an extended without using the fold-down bed extensions at the rear) it's a "short queen."
You won't really be able to put sheets directly on the bed. Not fitted sheets, certainly. You might get by with a "bed sack" like the ones they sell for travelers afraid of bedbugs, but that's really just a sleeping bag made of sheets instead of quilts, and I don't recommend that option.
I used a double-bed-sided sleeping bag for the first two years, and it wasn't bad at all. But my height just happens to be exactly right so that my body's pressure points while sleeping happened to fall where the bed had the most padding. Taller or shorter people would be much less comfortable.
About a year ago, I replaced my sleeping bag with a mattress pad. For me it's six of one, half-a-dozen of the other with regard to sleeping comfort. But it's infinitely easier to put sheets on a mattress pad and launder the sheets when the trip is over than it is to haul a sleeping bad to the dry-cleaners.
Engineering: Finding complex solutions to simple problems you didn't even know you had.