It's NEVER a bad idea to add a cooler unless you have one from the factory (in addition to the one inside your radiator). Believe me, TOWING a 6000 pound trailer is FAR different than loading 6000 pounds inside the truck (if your van would even carry that much), if only due to wind resistance and the extra rolling resistance of the extra tires
Take a look at the trans fluid temp chart below-it doesn't take that big a rise to do some serious damage. A cooler is well UNDER $100. A new transmission is well OVER $2000. Sounds like a transmission cooler has REALLY a good risk-to-reward ratio to me!