I hooked my truck up to a 20,000 lb trailer the day after picking it up from the dealer and didn't unhook it for 6500 miles. We have been buying new heavy equipment for over 30 years and have never worried about breaking equipment in nor have we ever had a problem with a piece of equipment that could be traced to improper break in. There is no way your dealer is going to refuse warranty on a truck because you used it to tow a trailer downhill 60 miles before it had 1000 miles on the clock.