My
Scamp 13' uses 1 7/8" and I believe the
Scamp 16/19 uses 2". I also believe all the Casitas use 2" also, so likely that's what yours should be.
On the underside of most couplers, there is an adjusting nut to move the clamp mechanism in and out -- Maybe yours just needs adjusting (a previous owner may have tried to use a smaller ball than 2" and adjusted it too tight).