The type of freon required is listed on your particular model of Mach 13.5 AC unit. This information will be listed on the chassis of the unit. If your unit is several years old, it will probably require R-22 freon. The newer models will use R-410A.
Remove the shroud to access the information for what type freon and how much is required for your particular Mach 13.5 RV AC unit.