that is very vague statement but perhaps I can answer.
on my thermostat the first setting is for fan - you can set it to "Auto", "Hi", or "Low". If you set it to high or low the fan will run nonstop (on the AC unit to circulate air) if you have it on fan, AC, or Heat setting.
The fan on the heater will still turn on and off as needed to maintain temperature.
the fan on the heater will also run for awhile after each heat cycle to cool down the heating element.
that's about the best help you can get without providing details such as heater brand, rv brand/model, pictures of thermostat, etc...