I dont go searching for lowest price when towing down the road. Generally you will find the prices near most interstates to be the same per state though sometimes higher near larger towns and cities. But while I try to stop where I can save more then a few cents a gallon (or avoid higher priced states if I can carry enough fuel not to need a refueling until the next state with lower prices) I just look for higher volume locations for diesel. I actully stopped at a shell that had good volume but probably not a lot of diesel volume as about 3/4 of the way home my filters clogged up.
All I could do is limp off the road and change them right then and there and continue on down the road. Thankfully due to this site I learned to keep a spare set of filters for the Monaco, and I now even keep a spare set for the F350.