Highway towing range hit is largely an aerodynamic drag issue. Any EV truck (or any car really, even gas cars have a big range hit) is going to get a massive ding in its range towing anything increasing it's aero drag even if it's an empty box. It's just with a gas truck you're starting with 300+mi often for a well equipped truck so you lose 100mi of range you're still over 200mi per tank.

But an EV, on a long range road trip you're rarely charging to 100%, you're often going like 5%->80% because the charging speeds fall off a cliff after a certain percentage. So you start off with maybe 300mi, but not really because after the first leg you're only using 75% of it, but now you're also using like 25% more energy because of the massively increased drag. So what was 300mi on a full charge became maybe 150mi on a full charge once you're on that second leg. Coupled with the fact what used to be free energy (heating the cabin with waste engine heat) if you're towing in cold weather you're not even going to get that 150mi.