The effect of towing on range is primarily determined by the aerodynamics of the trailer. Weight is a much smaller factor, unless the trip is mainly up and down hills, but aero still has the greatest effect. A good rule of thumb is a 50% reduction in maximum range for a trailer in the upper range of the vehicle’s tow rating, and that includes keeping speeds to 60 MPH or below.
This is definitely a case where YMMV (your mileage may vary) as each combination of tow vehicle and trailer produces a unique aerodynamic footprint. More trailers are being designed to be aerodynamic and it definitely pays to favor those when planning to tow with an EV.