The alignment shop that I worked with charged $125 to put it on the rack to check the alignment. And if alignment was needed that 125 would be included the final price. But they stated that the max price would be $400. But also said that the normal out the door price is less than $350. And if it needs nothing, the base price of $125 would cover all the base measurements. About an hours work.
They found they needed to adjust the rear axle parallelism. And camber on the rear axle. Final cost was $320. They did a great job.
It may be true that some trailer manufacturers do NOT align. But probably because they jig up the frame and spring hangars for welding. And the jig is set up to make locating the spring hangars very accurately. Other manufacturers may buy the frames with the axles mounted. So they supplier does the mounting of the spring hangers, instead of the factory.
People do get trailers aligned all the time. I am surprised the Jayco rep would ever make such a statement, that they don't recommend alignments. What happens when springs get replaced, or axles get bent due to curb scuffing, or overloading, etc. If you tires indicate the axles are out of alignment, you gotta get it aligned. Or you're gunna buy a lot of tires.