Or your choosing of product. May or may not help. The other suggestions made would be some options....I know Amsoil makes an octane booster too, don't know if it is recommended for regular driving, but worth looking into if running 87 octane to keep everything economical.
Keeping it cost effective, try a deeper pleat/aftermarket dry filter.
For me when driving/towing my 2014 Ram 2500 6.4/ 4.10 gears .. when empty I find that at 108Kms is the best highway fuel mileage and when in tow/Haul mode I keep the RPM's around 2100 and get the best mileage. I use K&N air filter with 0W40 full syn Pennzoil SRT oil and all other oil are synthetic too. You'll have to covert these #'s to MPG.. Best not towing was 14L/100Kms from Buffalo to Chicago (had about 2500lbs on board) and best towing is about 17L/100kms with 6 person bumper pull camper (older heavier one). I was very happy with those #'s but will probably never see them again due to the work been doing to the truck. I do not do as much highway and or towing as much anymore and that's mostly due to Covid but this truck spends most of it's time in the bush. We will not be talking about what mileage it makes in the bush ... lol but I can say that that aftermarket work that was done to the 66RFE helped with fuel mileage while off-roading but didn't do anything for highway/towing.
Large gas engine and 7000# truck is only going to do so well. Re-gearing? You'd probably be looking at 500,000 mile payback period. Depending on your driving style it well may never pay off. Cleaners and additives? You'll burn more gas driving to buy them than you'll ever save. If it matters slow down. That's by far the biggest thing you can do to save fuel.