My truck towed our old hybrid just fine without struggling, but when we upgraded to our heavier 24FBS, I felt that having a transmission cooler wouldn't hurt. Coincidentally, the truck was also due for a transmission service, so I just took it to a reputable transmission shop and had it all done at the same time. It cost a few hundred dollars including the service, but if you're handy, you could probably do it yourself. It's a bit beyond my skill, so I paid to have it done, and they put in a stacked plate cooler rated for 16,000 lb GVW. It's probably not 100% necessary if you're towing well below your limits, but finding out you're wrong would be a lot more expensive.
Geoff & Jill
& Sierra, the little white monster
2013 Ford F-150 XTR SC Ecoboost
2015 Jayco Jay Flight 24FBS