Your situation is one of two things:
1. Compressor cycles on / off and the blower fan continues to run, which is normal operation for most units that have controls at the ceiling.
2. The ambient temp in your RV is still warm enough that the A/C is not keeping up with the cooling. (normal operation)
Some wall mounted thermostats that control a roof A/C have a Fan Hi / Low setting and a Fan Auto Hi / Auto LOW setting.
On the AUTO setting, the fan will turn off when there is no more demand for cooling.
TO give a better answer to your qiuestion, we need to know more detals to your set-up:
What brand / make / model A/C do yo have
Where is it controlled (at the ceiling panel or via a wall t-stat
If it's controlled by a wall T-stat, post a photo of it.