Just a guess here, I will admit, but the idea of lippert manufacturing their own steel is almost zero. Structural metals comes from mills that cost hundreds of millions of dollars to build.
I just looked online and found 12" rolled beams for sale, and the spec of the material thickness is .232", so yes, 'dogs measurement of 1/4" ( .250" ) would be within twenty thousandths. Sounds right to me.
As I stated earlier, the 6" beam under this 19' trailer I am in right now has a material thickness of approx 1/8" ( .125" ). Seems reasonable to me.