They tried to put 20 inch wheels on my F350 when I ordered it. First I did not like the low profile sidewalls. It's hard to believe they with deal with rough roads as well as the 18 inch.
And the deciding factor was when I checked the price of replacement tires. There were few available types and they were MUCH pricier. That may have changed in the intervening time.
BTW it took a month to get our ordered truck, when we ordered it the dealer told us 6 to 8 weeks. I got the idea that the dealers do not want to sell you an ordered vehicle, they want to get one off they lot which they are paying interest changes on. I tried to get competing prices on our truck from different dealers and never got a reply. But when the truck came in the salesman told me they had calls from 6 different dealers in 3 states trying to get it sent to them.