We have this, or something very similar to it, from Overstock.com.
We cut the straps off and, by day, it folds up into a large pillow cover and lays over the tool bag, Dutch oven, my crochet bag, etc., which rest between our two seats when we are traveling. Lily curls up on top.
. She likes to be close.
Used to have
to ride in my lap, but at 50lbs she is just too big and heavy for that. Like wearing a bear rug.
All year round.
We have one of those thin, all-season down comforters with a duvet cover on it, as a top cover. It is perfect, and truly seems to adapt to whatever the temperatures are. By day, it folds up tightly and goes into that little area under the bench that the hot water bypass is behind.
We don't use sheets since we got these items. So much easier to stow and less to get shifted around during the night. We wash the mattress pad and duvet cover every week. Sanitary enough.
We put our bed pillows into shams we had made out of washable, sueded black fabric. They ride on the benches during the day.
We love our bed in the Interstate. Very, very comfy.