We have the 2012 30' VLite with the 15k AC. We have only been on 2 trips since the purchase but don't have any real problem with cooling even being in south Mississippi's humidity and mid to high 90 degree temps!
We try to find a site with shade of some sort and use a large oscillating tower fan in the rear living area and a small box fan in the front bedroom(at night). The AC runs on high fan all day and at night we'll turn it down to low fan. We try to not leave the entrance door open any more than needed and also keep the shades pulled down. I can't say what the inside temps are with the thermostat set around 73 but I don't like to be hot. I'm on vacation!
The combination of fans and shade seem to make a difference. Airflow from all the ducts seem to be fine. You may have an obstruction in one of your ducts or you may have to modify the duct opening. There has been a previous thread on this. The upgrade to the 15k from the 13.5k may be a worthwhile investment. After all, you need to be comfortable.