13.5 AC should be fine. We have 13.5 AC unit in our 30 foot Salem by Forest River. We've traveled for 10 hours in 90+ deg heat across GA several times. Yes...when at destination it takes an hour to cool it off...but its a box that's been in the sun. However, once cooled down, the AC does its thing and keeps us cool via the auto thermostat (turns off/on based on temp).
Point is...I worried about 13.5 vs 15. 15 would have been great, but we are happy with 13.5. We've even camped in 100+ temps and it did okay.