It's really simple for those who don't worry about every cent and like convenience.
Truck stops are next to the freeway and easy to access. Simply have the wife go inside to prepay with pump number. Tell attendant Diesel amount you won't exceed and DEF if needed and amount not to exceed. While you are fueling she can shop, get some food and use the facilities. When you are done call her cell and let it ring twice and hang up. This tells her to go pay. You pull forward and go in to use the facilities and then you both head right back to the freeway. EZ CHEEZY.
Modern Truck Stops are VERY nice and have food that is from healthy to not so healthy.
I carry a 5 as a backup, used it once. Not wasting bed/toolbox space for additional tank.
If I did anything I would replace 32 gallon tank with a TITAN 55 gallon replacement tank.