Transmission cooler for 99 2500 gas (trans gets hot)

[Bacanak]

My ‘99 2500 gas Magnum V8 truck, automatic transmission, regular cab, steel flatbed, transmission gets hot after driving a while when I haul a horse trailer up grades (not even very steep grades), otherwise does fine when hauling. I want to put a better tranny cooler on it, it has one but it looks small. Should I have one put on with a fan or without a fan? Currently has no fan on the cooler. Current cooler is mounted on front of the A/C condenser. I’m not a mechanic so need some advice as to what to buy.

Cheers,
Cheri