Rubber cost would be my biggest issue with 20" rims. The ride quality will also suffer slightly. I would ask if the dealer would swap out to the 18's.
I thought I would check and see if 20's are more expensive and surprisingly they are the same price on Tirerack. Within 38$ anyway, but the options for 18" tires are better. Way more models available for 18's.
2014 Jayco Swift 281BHS
2015 F250 XLT 4x4 Crew Cab, Short box, 6.2 gas
Solar powered! The trailer not the truck!