The best litter box mod I've seen was to a Class C with a full basement. They installed an almost invisible pet door through the inside step well into an adjoining 18x24" pass-thru compartment. The litter box stays under the floor out of sight and smell. When the litter box needs changed it is removed through the outside compartment door eliminating any mess inside. My Class C has the same setup but my wife is too afraid of her cat escaping accidentally so we don't take her camping.