As mentioned, knowing where and when to buy fuel is a major factor. The price of the fuel is not the same as the cost of the fuel in a truck the size of a Cascadia. IFTA has to be figured in for whoever is paying the fuel tax. I wrote an article explaining this that used to be pinned. I'm not sure where it's gone now. My quarterly IFTA bills were like $7.43, $3.82 etc. while most people I know were getting bills in the $1xx-2xx and as high as $4xx.
Another good point is buying fuel 50 gallons at a time as a solo so you get more shower credits.
Once you know where and when to buy fuel, the 60% side is probably your friend, if you are a good driver for MPG. If you are content to run 62-64mph even when it's posted 70 or more and if you follow 15-20 car lengths behind not 15-20 feet behind you're a good 60% candidate.
If the truck owner is agreeable to let you remove the visor above the windshield and install the mudflaps that are slotted you'll gain about 1/2mpg which is a major improvement. Keep the tires fully inflated. Believe it or not, keep the bugs washed off and get a full truck wash as often as the owner will buy one for you. It may only make a tenth of a mpg difference but that can be 200 gallons of fuel per year.
Keep good records of where you buy fuel, the weather and road conditions of your trip, date, cost, personal notes and before long you'll have an excellent journal and be ahead of almost everyone else on the road in wise fuel buying. Good luck.