My Ford dc-to-ac inverter sometimes just does not come on.....orange LED light off no output.

Then next time it will work fine.

Less than 100 watts of load: macbook charger.

Speculate: Is battery/power management system commanding the inverter to stay off for some reason, or is the inverter flaky?

 

Inverter is flaky, or the inrush current of the device is tripping the overcurrent protection. The inverters in these trucks are nothing to write home about.