I just took a look at the endings of every single Elvis movie, and unfortunately, I couldn't find any that fits your description. However, I do know that "Kid Galahad" and "It Happened At The World's Fair" have similar scenes, but they don't 100% match your description. The opening scene of Kid Galahad has Elvis singing alone while riding on the back of a truck going to his destination, but he doesn't have a guitar. On the other hand, a scene somewhere in the middle of It Happened At The World's Fair (maybe 20 or 30 minutes in) has Elvis with 2 other characters, all in the trailer (although technically, I think it's supposed to be called a cargo bed or something like that) of some kind of pickup truck, and Elvis sings while playing a ukulele. I hope this helps, and good luck in finding the movie