Coolant over heating

My Dodge Ram 1500 5.7 hemi 24000 miles is over
heating. It runs hot all of the time between 219 and
230 degrees. It red lines with in ten minutes of
towing reaching 250 degrees. It even red lined on
the freeway last week in 80 degree temps. I have
replaced the radiator cap, thermostat and the
dealer said the fan clutch was bad and replaced it.
Still no difference. Because nothing shows on the
computer diagnostics the dealership is telling me
there isn't a problem. It is still under warranty. I've
owned the truck 3 months and purchased it to tow
a small trailer. I can't even use it to tow without red
lining in the first 20 minutes. Super frustrated!