No matter how well you take care of a vehicle, it's going to wear out sooner or later. Unless you tow this trailer regularly, year-round, and this change was sudden, you can really only attribute it to normal wear and tear. 4-6 months go by between the last trip in the fall and the first trip in the spring/summer, and you put a lot of miles on the truck in between. It could have been right on the edge last time you towed, and just enough wear between now and then, that it doesn't have enough power for the hills anymore.
Don't put much faith in having things "checked." They don't have x-ray vision. They only do a cursory visual inspection, and if nothing jagged is sticking out anywhere, it's declared "fine."
Also be prepared to put a transmission in soon. Unless you did regular fluid changes at the proper intervals, you would've been better served by leaving it alone. An "oh ****, I haven't changed this in 200,000 miles" flush and fill usually kills the transmission.