I agree with Taxman Hog. You should tow in the tow mode. The braking system is enhanced, the cooling system is enhanced if you have the max tow package, the trailer brake controller will not work in normal mode, hill decent control is active if you have max tow, the motor is optimized in tow...
You would know that you have hill descent control as you can toggle it on/off. It's not just an off road function.
You just don't want your brakes to go out (overheat) or pick up too much speed going down a long, steep hill while towing. Regenerative braking has a similar effect. One more...
Hill descent control is like a cruise control for going down hills. It automatically engages the brakes. It's not as important for a lightning which will use regenerative braking (not use the actual brake pads) to reduce speed when going down hills as compared to ICE vehicles.
You will likely have a capacity of 5k (standard range battery), 7.7k (max tow with standard range battery or long range battery), or 10k (long range battery and max tow package).
edit: There is a reduction in towing for the Platinum trim.