I am not exactly sure of the size you would need(I would think a 20-22ft would work), but I would recommend a PolyPro III cover, we had one for our popup and also for our X17Z. I plan on ordering one soon for our Starcraft.
2013 Starcraft Travel Star 229TB
2011 Jayco X17Z(traded in)
2010 Jayco Jay Series 1207(traded for the 17Z)
2011 Chevrolet Silverado 2500HD Z71 CC 6.0L/3.73
Reese "Pro Series" 800lb Trunnion WDH w/sway control