Double ditto what Frank said.
A few years ago, we went for a short day trip on a bright sunny day and left the vent over our bed open. Well, that pop-up thunder shower centered itself on our campsite and drenched the bed which (of course) is right under the vent.
That did it, off to the RV store to buy a MaxxAir. My budget was tight that year so I only did the one over the bed. The year after, I drove by an RV shop with a spare $20 in my wallet and came home with another. Last spring I finished off the third one.
If your budget allows, do 'em all at once - you will never regret it.
Now to save my pennies for the vent fan upgrade....