We had a new boat that did the same thing once and the explanation we got was this.
They wax the fiberglass mould every half a dozen boats done (can't remember exact number) which helps them release better. If you score the first boat out of a freshly waxed mould, it's safe to say you are going to have yellowing problems. This is regardless of if it is in the sun or not. But the sun does amplify the problem.
If you get a boat that is one if the last before the mould is rewaxed, then your problems will be limited.
I would thing the jayco's work off a similar principal and it explains why some so it and some don't. Polishing helps but it's a hard slog.
Luck of the draw really, but still disappointing. Mine lives in the sun and heat and is still fairly good after nearly 2 years