Driving 10km slower for gas mileage

it may depend on your truck gearing. On my 2015.5 duramax, towing at 55 I will be in 5th gear for sure in tow/haul mode and likely even if not. At 60mph I'm in 6th when out of tow haul, 62 goes to 6th in tow haul. that extra gear pretty much evens out fuel mileage. Since wind resistance, engine rpm, and tire rotational losses are all square law function vs speed, they will eventually dominate over linear losses, rolling resistance etc. But my experience is that those "square law" losses are balanced by the linear losses up to the 50ish mph range. by the mid to upper 60's they really dominate. But in the 50-low 60 mph range not as dramatic effect on mileage.

you can try but may find the difference in mileage not noticeable enough to be worth the effort.