I don’t think this is true at all. If towing the trailer doubles the energy required at the wheels (and both engines maintained the same efficiency under the additional load) then both ranges would be halved.

In practice, diesel engines can become more efficient under additional load, and it obviously varies between models but it’s not a 16% vs 50% difference like your example.

The reason EVs don’t tow well is they don’t have a high enough range to begin with. Diesels have nearly double the starting range of an equivalent EV, and storing additional fuel on board is an option.