I would assume that it's a 2-5/16", most campers that I've had were except the pop-ups.
If it's accessible I would check to be certain but I'm fairly certain that it is a 2-5/16" ball.
GulfStream Conquest 19RD - 2021
Kia Sorento - 2016
Kawasaki Voyager XII - 2001
Starcraft Pop-up - 2006 (being used by kids)
past campers: Coleman Santa-Fe Pop-Up, Thor Prisim 2000 TT, Dutchman Calssic 5th wheel, Jayco Eagle TT, Time Out (motorcycle pop-up)