I had the same issue happen whenwe went to turn furnace on for the first time in our new 2012ST25RB. Easily remedied via the one controlling thermostat. I think its set up that way with the A/C fan kicking on by default. If you read the thermostat instruction booklet that came with the RV, it tells you exactly how to disable the A/C fan coming on. I think if my recollection serves right, the thermostat fan is set to "Auto" which has fan come on automatically. If you can't find your instruction book, let me know and I can quote the exact instruction from mine. You should have received all the instruction manuals for all the appliances, radio, tv, etc. when you picked up your trailer from dealer.
Take care!