As mentioned elsewhere, I sleep in a double-width sleeping bag (actually two bags zipped together) in Gertie's queen-sized overcab bed. The bags are nylon on the outside and flannel on the inside, which is great in cold weather but can be a bit warm in the summer. Many RVers use a product called Travasak, which is essentially two comforters sewn together—much like my sleeping bags—but with a removable bedsheet liner inside. Besides feeling more like a "real" bed, the removable liner can be laundered separately. Travasaks are not cheap—$230 for a queen-sized outfit—so I decided to make my own sheet insert and use it with my sleeping bags.
I bought a couple of twin-sized topsheets (flat sheets). This may sound surprising, but I had measured the inside of my sleeping bags and found that they were only 52" wide, so I wanted to get as close to that width as possible. Even so, the twin-sized sheets were 66" wide, so I had to cut off a foot of width. (I left a couple of inches as a seam allowance.)
After hemming the cut-off edge, I sewed 30" strips of hooked Velcro up the bottom sides of one of the sheets, as shown here. That was to hold the sheet insert in place in my flannel-lined sleeping bag, and it worked like a charm. Why 30"? Because that was the length the stuff was sold in.
Then I put the sheets right sides together (Velcro inward) and ran a line of stitching across the bottom and up each side about as high as the Velcro strips (30"). After turning the resulting pocket inside out, I had my finished bedsheet insert. The hooked Velcro holds it nicely in place—the flannel sleeping bag lining acts just like looped Velcro—but it's easily removed for washing. Sure beats having my sleeping bags dry-cleaned! And it's much more comfortable. In hot weather I can remove the bedsheets from the sleeping bag and sleep in just the sheets. It's a good, inexpensive all-around solution.
If you want to make a complete Travasak-like outfit, you can buy a pair of comforters and sew them together at the bottoms as shown here, then make a bedsheet insert to match. Unless the comforter material lets the hooked Velcro get a good grip (as in my case), you'll probably want to add a couple of strips of looped Velcro to the inside of the bottom comforter to mate with the hooked Velcro on the sheet insert.
When shopping for comforters, don't make the mistake I initially did and buy too large! A queen-sized comforter does not necessarily fit a queen-sized bed. Measure your bed's top area and choose accordingly. Unless you have an "island bed" where the comforter can hang down, you'll probably find that a twin-sized comforter fits a full-sized bed, a full-sized comforter fits a queen-sized bed, and so on. Otherwise you're going to end up with a lot of material bunched up at the edges.