I am towing a 29' camper with my 1500 5.2. Is it better use synthetic fluid or install an exterior cooler? Somebody told me synthetic fluid will lower the temp significantly more than an exterior cooler.

 

the atf+4 is already full synthetic.
yes, you should put on the absolutely biggest cooler available.
unless you've already had it rebuilt, these transmissions are the weak point on the truck.
try to keep it cool and the fluid changed regularly.

 

Have it changed at a shop that uses BG Products, there fluids are more advanced and some of the best made.
http://www.bgprod.com/products/transmission.html