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.