Most caravan beds are made up from the two facing sofas at the front or rear of the caravan. When making the bed you will find it most comfortable if you turn the cushions upside down as it helps remove the humps. However, this still leaves an a bit of an uneven surface.
An alternative to this is place one of the back cushions on its edge to form headboard. The second back cushion is then laid in front of this in the conventional manner to support the pillows. This removes the issue of the caravan cushion humps being in an awkward place.
The two seating cushions should fit exactly to the remaining bed length. Fit them lengthwise with the raised edges running down either side of the bed.
Also if you get drafts from vents in the caravan. Placing a blanket on top of or underneath the cushions can help reduce this.