Is there any advantage to having 20-inch versus 18-inch tires on your RV
The difference is the 20 inch rims your option is only the low-sidewall summer/sport tire. On the 19" rim, your option is the higher-sidewall all-season tire.
The all-season tend to last longer. The 19" with the higher sidewalls are less prone to pot-hole damage to the rims and tires, and the ride is less harsh. I had 20" sports tires on mine, and in the first 4 months, bent all the rims, and ruined two tires hitting potholes. Since switching to 19", no further damage, and a better ride. The 20" may be fine for the well maintained autobahn, but on bad surface streets, not so much.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 19 inch 427 wheels, EVoInnovate EVSE