Most RV's only have 4 seat belts but it may be possible to add a 5th.
My rule is "Buckle up or transmission remains in PARK".
I have seen far too many accidents, either live and on camera (remotly controlled traffic cam) in evidence photos (Taken by police officers at the scene) or the car after the crash. Where people failed to buckle up.
My daughter's first babysitter (Back over 3 decades ago) had a split lip, I ask about it.. Her 3 friends laughed at her when she buckled her belt.
She then attended 3 funerals (Theirs) because the belt saved her life.