We used the "miniature" tennis balls which come in a 6-8 ball pack. These balls are the type a lot of folks purchase for dog toys. They worked prefect after i cut an X on the end and carefully wedged them onto the rain gutters. Just take some care when putting the cover on so you don't inadvertently dislodge the balls.
Days camping (2015)----88 days
Days camping (2016) ---110 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi
30 years RV'ing
12 different RV's