I replaced a 13,500 with a 15,000. You need to get the model number of the 13,500 and the thermostat that it uses. Send an email or call Dometic and they will tell you the model number for the 15,000 you will need. You more than likely will also have to get a new circuit board which Dometic will give you that part number also so it will interface with your existing thermostat. Be sure and get the dip switches set in correct positions. I also added soft starts to both units.
Some people are changing there units to a Houghton sold by Rev and say it is much quieter plus about 1/2 the price.