You know how "Pink is for girls and Blue is for Boys"? Well, use color-coding for your RV trips as well.
Make sure everyone has a chair; towels and whatever else there are duplicates of, in their own color. If you don't want chairs or other items in every color of the rainbow, use plastic zip ties or colored electrical tape to mark everyone's chair, floatation toy, or beach bag. Not only will it save a lot of argument's over "whose it is," it will help you identify at a glance who needs to be picking up their things!