It shouldn't need freon unless it has a leak, and then the problem is to fix the leak before re-charging. Further, RV a/c's are not made with recharging ports - you would have to pay a tech to add one to recharge, and you would also have to convert to R134 or other new refrigerant if it is old enough to have actual "freon".
Long story short, if it needs freon, it's probably replacement time.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL