I had a 30' fifth wheel with a 13.5K unit and when it was fairly new, I always thought it did fine, but the longer I owned it, the more I realized it was just wishful thinking. In the extreme heat of Summer, if you aren't in the shade, 13.5K is going to have a tough time keeping the trailer under 82* maybe even 85* depending on how well the trailer is insulated. I would definitely spend the extra money for 15K.
If you can't get it from the factory with a 15k, the dealer is probably going to ding you pretty good for a swap. Their biggest issue is that they are stuck with what is considered a used unit to sell deeply discounted because it is used. I would think best case is that the dealer would do the swap for the cost of the new unit and give you enough credit for the smaller unit to cover their labor. You could always just pay for the new unit and labor then sell the 13.5 privately which would probably be the most bang for the buck.