The "fart fan" (6" plastic blade) is a push on, friction fit fan blade. The hole in yours may be slightly enlarged causing it to slip off the motor shaft. You can either use some silicone, gorilla glue, super glue or even a couple of strands of copper wire from a cut piece of an old extension cord. Any of them will "tighten up and hold" the blade to the shaft.
You can buy two replacement blades for $10 on Amazon:
https://www.amazon.com/Blade-Replace...-1-spons&psc=1 or, you can use that $10 as a "down payment" on a better fan.
Personally, I shy away from HENG fans as they make far more noise than either Fantastic or Maxx Air vent fans. Part of the HENG noise is installing the blade and motor "inside" the RV. That is not the situation with the more expensive replacement vents where the motor is "further up inside the vent dome" moving the noise outside the RV by keeping it above the ceiling rather than below the ceiling.
Anyway, if you slip a couple of "SINGLE" strands of copper wire through the hole and push the existing fan blades back in place, they should stay... At least until the replacement fan arrives and you toss that "itty bitty fart fan" in the trash.