I like the idea of a spice rack with one or two shelves! You could pre-build the box out of 1/2" plywood - make it 1-1/2" wider than the opening and 2-3 inches deep. Attach some 3/4" x 1/2" trim to the face on the inside of the box. Stain & finish it to match your cabinetry. Remove the top drawer and slide the box edgewise into the space through the opening. Then use Gorilla Glue on the face edge of the molding to attach it to the back side of the cabinet frame. By putting the molding on the inside of the box, you can use quick clamps to hold it in place while the glue dries. Or you could use a brad nailer to nail it through the face of the cabinet frame; then fill the holes with a hole-filling crayon that matches the finish on your cabinetry.
In an RV every bit of storage space counts, and (like you) I hate the idea of all that unused space. Whatever you end up doing, please post pictures when you're done. We'd love to see what you came up with.