ac/dc inverter will not work in 2011 GMC van

I put in a new Schumacher 1000 watt Ac/Dc inverter that has a high voltage safty that shuts it down at 15.5 volts. The new 2011 van, which I put it in, also has a fuel energy saver which shuts the alternater off and then at times will spike up to 15.5 which in turn will shut down the inverter. Sometimes it stays at the 15.5 volts most of the day, which means the inverter won't work. Is there an easy fix or has anyone else ran into this problem. The dealer checked the charging system in the van and it is all normal.