My 5th wheel is 27 feet of box plus a ladder too. I ordered the 27 foot cover and it fit fine. Plenty of slack to cover ladder plus spare tire.
Would recommend that you get some swim noodles or semi rigid pipe insulation. I covered the ladder as the sharp corners of the outer edge of the ladder would have caused tears in the cover. Also used at sharp places like folding steps, corners and rear bumper ends. Don't forget to get wheel covers too, to protect tires, as cover will not reach all the way to ground. :)