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, 300W Solar!
2015 F250 XLT 4x4 Crew Cab, Short box, 6.2 gas