Perhaps if we knew what you plan to tow/haul and where you want to travel with your truck, we could provide better suggestions. If you plan to stay in the plain states where there are not too much elevation changes or height, you can get by with a smaller engine. If you plan to visit the mountain ranges to either side of you, a small engine could do the job as long as it is on occasion but will struggle. I climb at least 3000' to go east or south with a GCW of 18-20K lbs four times a year and would not enjoy that drive without my diesel. The trips I make weighing under 12K lbs at sea level-1000' could easily be handled by the smallest gasoline engine offered.
BTW: I will be coming down I-29 through IA next week and that will be a breeze compared to crossing the Continental Divide.