Why does my A/C fan on when I run the furnace?

If you have A/C heat pumps they will heat and blow air through the ceiling ducts. Newer RVs the heat pumps are interconnected with the furnace. When it is too cold for the heat pumps to maintain the set temp the furnace will come on. At times mine cycles back and forth between the A/C heat pumps and furnace. Not sure which system you have. Posting a picture of your thermostat would be helpful.