In 2019, I took a 9000 mile road trip, towing a teardrop trailer. The only trip planner I could find that could adjust for towing was EV Trip Optimizer, which has a slider called “power factor” that allows you to adjust for the extra load of a trailer. I knew that the baseline for 65mph in my model 3 was 240Kwh per mile. so I towed the the trailer at that speed and found that I was using about 400KWh per mile. With a little experimentation, I was able to reliably predict my range for each segment of the trip.
Sadly, EVTO has been surpassed by other options, and will cease operations in July 2024. I now use A Better Route Planner, which has a “Reference consumption” setting, that allows me to enter the 400Kwh per mile setting. Works well for me, your mileage may quite literally vary.