I am not exactly sure about the RV situation versus Car. But if there are no difference. the rule are pretty clear in CA :
"Your child must be secured by either a federally-approved child passenger restraint system or a safety belt depending on their height and age.
Children under 2 years old must be secured in a
rear-facing child passenger restraint system unless the child is 40 pounds or more, or 3 feet 4 inches or taller."
Special post for your reference :
https://rvshare.com/blog/things-need...eat-belt-laws/