You would need an extra trailer to pull the fuel you needed to cross the country. And it would take a lot longer through the mountains because of the reduced power output. A friend learned that the hard way when he bought NG powered van. He eventually converted to gasoline.
LNG tanks mounted on the truck give comparable ranges to diesel. Nobody is driving an 18 wheeler coast to coast without refueling along the way.
This isn't a theory. Several Long-haul operations have been using them for a while.
Only if you start with an engine designed for a different fuel then feed it natural gas. A properly designed engine like Westport is putting on 18 wheelers and the like is not low power. It is a more expensive engine than the same horsepower diesel.