Hang curtains. A basic decoration for a pop-up camper is curtains. Hang matching curtains on all the windows. These will not be affected by the top coming down.
Add fabric to the interior pop-up walls. In lieu of wallpaper which will ruin when crumpled, use a light, decorative cotton fabric. Cut the fabric to fit the wall space. Glue to the wall with epoxy glue at the top, around the edges and the bottom to hold in place. The fabric will act as wallpaper and will collapse with the wall without damage.
Paint the cabinets. While hanging pictures on the walls is not possible, painting decorative scenes on the cabinets is. Use stencils and oil-based paint to create a mural or just add decoration.
Lay throw rugs on the floor. Carpet may become musty or ruined during camping outings. Pick throw rugs to match the wall and cabinet colors. Place in the walkways and any open spaces.
Place decorative pillows on seats and ledges. Colorful pillows will add to the decor and comfort of those occupying the camper.