Effect of Tire and wheel size question.

I am ordering a new truck( F250) with camper pkg for an Artic Fox 811. (Will be loaded very close to factory limit or slightly over) I did a limited search but could not find much discussion on tire and wheel sizes effect on handling and mpg. I will only be carrying the camper about 5% of the time but want to choose the best factory option(18 or 20inch) or maybe get after market wheels and tires. Thanks in advance, Doug