Add my vote for yes. Can't be without vents for all the reasons mentioned above. Put in more than you think you need, you won't regret it. If you think you need one, put in two. My converted "toy hauler type" trailer has one in the center, one in the bathroom, plus the a/c, and I really wish I had put one directly over the bunk. Going to go back and add one more. Really nice to get a little air moving on a warm night, and to let the day's heat out. Make sure you use the covers like Bob said, MaxxAir is the common brand name, but there are a bunch of other brands that do the same job. Let you keep the vents open in the rain and while moving. Plus every trailer I had without the covers eventually had a lip blow off the vent while travelling, the MaxxAir covers cure all that. Figure $25 to $35 each depending on where you get them (every rv store has a few in stock) and 15 minute install.
The vents themselves are kind of a pain to put in depending on your roof structure as you may have to add bracing. The vent itself is easy, it's preparing the hole and bracing. The hole needs to be framed all the way around with metal or sturdy wood for the screw to fasten to, and to keep the metal on the roof rigid to maintain a seal. Use the sealer tape they have at the rv store under the flange, plus a bead of rtv silicone around the edge and on top of the screw heads and you will never have a leak to worry about. And with the covers wind and rain will never make it's way in.
Also, they have some vents with a fan, and in my experience the ones with the tiny approx 4" diameter fans are not worth wasting your money on, they are noisy and don't move much air at all. If you want a fan, bite the bullet and get the full sized fan, they are pricey but you are getting what you pay for.