I understand your desire to have a nice looking bed. If you just throw a sleeping bag on the mattress then you have the sides of the mattress showing. Not very nice looking.
Our solution....A nice fitted cover for the corner mattress that is color coordinated to the upholstery in the Motorhome. It was challenging to install but looks very nice. Then we purchased special bedding. Its queens sized and like two sleeping bags zipped together but has sheets that Velcro inside. One side is light weight (Summer side) and the other side is heavier (Winter side). The bedding is a different color but it also coordinates with the interior upholstery. We then have some pillow shams that match the upholstery. It all looks very nice and is quite easy to make up. Takes two or three minutes to make up the bed.
We remove the sheets as needed during the camping season for laundry and we have the covers dry cleaned once every season or every two depending on how much they are used.
So far it works for us and we have perfected the art of "get out of bed over your partner in the middle of the night" ;-).
Hope you can find a solution that is less burdensome and makes your travels more enjoyable.