Man, am I ever glad to see your post! I've had the same problem since I bought this MH last year. Recently I posted on another thread when somebody complained about a noisy fan or something else under their bed. I replied that it could be the inverter. Somebody else answered up and said that it was definitely the converter. Well, I had already verified that it was definitely not the converter but the inverter as that's where the box where the air was coming from.
Sounds like the folks at Xantrex weren't very helpful. The fan comes on for no reason every so often just 'because'. I've experienced the same thing and I know there's no draw on it and it's not too warm either in the unit or outside air. It does this year around no matter the temperature inside or out. And yes, it's rather annoying.
As for what to do about it, guess I don't have an answer other than to open the box and clip the wires to the fan. And that may not be too wise as it could get warm with use. Can't see a good answer from here