You guessed it, OP. Never mention "trailer" to Wal-Mart. I took two tires/rims in to have belted radials put on for my little 10' utility trailer. They said they would not mount the tires.
I waited until the next day, took the same rims in and told them it was for a sand buggy I had. They mounted with no hesitation. They have gotten in the CYA mode so heavily that you can barely get them to do anything anymore. I'm waiting for them to open a self serve car clinic like the self serve check outs they are pushing.
BTW, I don't use their self serve check outs unless they are willing to, either, give me a huge discount on the products I'm purchasing or until they put me on payroll for the period of time I would be "doing their job".