I'd call National and use their recommendation. Then it is their bad if it doesn't fit right. Mine is, as their specifications state, a "Class B RV Covers - Fits 28' Long 342" L x 84" W x 96" H Color: Windsor Tan." I think they do 26'-28', 28'-30', etc. It is plenty long on the sides, and would probably handle your extra foot in length. I would not complain if mine was a bit tighter.
Only down side (for some folks) is that the side zipper is between the passenger door and the coach door, providing access to both, but not as convenient for the coach door, but still doable.
Dougn