Real test....
Mount a class 3 hitch, and hook up an 8000 lb camper, and see how far it will tow it at 65 mph, on a charge.
F-150 pulls the same amount on a tank of gas. Then fill F-150 and continue on.
Plug Tesla in and see how long to recharge, than continue.
Keep repeating for 2500 miles (basically across USA) and see who finishes first.
F-150 is rated for 8000 lb towing, so Tesla should compare apples to apples.
Grumpaw
The F150 is rated to tow as much as 13,200lbs in the proper configuration. 8k should be a piece of cake for just about any F150. According to the following link, a modern ecoboost F150 with a 10 spd trans gets about 8.7mpg while towing 7k + trailer, so the truck could theoretically drive about 240 miles on a 30 gallon tank with your hypothetical 8k load:
The Fast Lane Truck conducted a real-world F-150 fuel mileage towing test between the 5.0-liter Coyote V8 and the 2.7-liter EcoBoost V6.
www.motor1.com
As for charging, Tesla's newest Superchargers can charge at a rate of 1000 miles/hr, or 75 miles of charge in 5 minutes. That means a top level Tesla truck with 500 miles of range (unloaded) that might get 250 miles while towing, would have to charge for about 30 minutes every 250 miles (or about every 4 hours if you drive 65mph). Most people are ready for a bathroom break, meal or chance to stretch their legs anyway every 4 hours. And Tesla is upgrading chargers all the time, so what's available now might be very different from what's available in 3-5 years when this truck comes to market.
So the trucks could probably drive about the same time and distance between fuel stops, and the time of the stops would probably favor the f150 (for now), but it's not a huge difference. And before anybody says "but where are the chargers!?" here's a map of the Tesla specific superchargers in North America:
There aren't as many as there are fueling stations, but if you've got a 250 mile range, you can easily get from 1 charger to the next. This map doesn't show other non-Tesla specific chargers, so you're likely to have many more charging options in the hypothetical scenario you posed.
So in theory, the F150 saves you about 15-20 minutes per fuel stop if you don't eat at the same time, etc. If we assume a 250 mile range for both trucks, A 3000 mile journey across the country would require 12 stops. That's a maximum savings of 4 hours over the course of a 3000 mile trip. But in reality, the Tesla skips the first fuel stop because it would leave home fully charged, and it could possibly charge overnight in other situations (assuming you stop and sleep). So the F150 might save you a realistic 2-4 hours over the course of a 40 hour drive? 5-10% faster, maybe? Less differnce if you eat at some fuel stops, or charged the Tesla over night.