We have been okay with just the 13,500 BTU unit in our 2015 23D International Serenity (pre overhead ductwork). The 15,000 BTU is a drop in deal using the same opening and the same size circuit breaker so wiring is compatible. However, the hole in the wallet is less easier to fix.
WBCCI Life Member 5123, AIR 70341, 4CU, WD9EMC
TV - 2012 Dodge 2500 4x4 Cummins HO, automatic, Centramatics, Kelderman level ride airbag suspension, bed shell
2014 31' Classic model 30 twin beds, 50 amp service, 900 watt solar system, Centramatics, Tuson TPMS, disc brakes, 16" tires & wheels