If we have plenty of large jalousie windows, and a window unit a/c, is it worthwhile to add a roof vent/fan? My main concern of omitting it is when we cook something smoky inside, the highest exit for smoke (if we have no vent) would be the door.
However, leaving out the vent would make our roof completely solid, eliminating potential spots for leaks. We don't have a need for any other vents (no onboard waste water tanks or gas appliances). Maybe I'm being overly paranoid about this, but I see lots of water damage on campers around the roof vent (looking online), and ours had lots of rot around it as well.
I think a roof vent is very important for the reason you mentioned. I install a Fantastic Fan in every trailer and they really help. If proper installation is done on the vent (putty and Dicor self leveling lap sealant to start) and maintenance yearly, they rarely leak. Also, I shim the vent frame higher than the surrounding roof, from 1/4 to 3/4" so that water drains away from the vent.
Dicor sticks to itself, so you can touch up the flange seal whenever necessary.
You pose an interesting question here. Absolutely any hole through the skin, especially the roof, poses a potential leakage problem. As mentioned, proper maintenance is the key to prevention. There are models that were built with no holes through the roof, although they are few and far between. I have the remains of a Bravo that was built like that, and I think the Shasta Loflytes were also roof-vent-less. It's been my experience that the vents are quite valuable for airing out though and I am often grateful for mine. I don't have the Fantastic Fans, and the convection of warmer air still changes the atmosphere inside in a hurry.
Loliners often came ventless as well. It kept them low enough to fit in a garage with special "lowering" wheels.
I would frame and shim for a vent regardless if you install one now. On my Loliner, I drilled tiny 1/16 holes in the ceiling to mark the cut out, easily done with a multitool. Also, run 12 volt wires to the vent area from the Kitchen cabinet. You can add a J box inside the cabinet to house the wires.
As you face the front of the trailer, there should be an indent cut on the left side toward the front. This allows you to use wing nuts and tuck them out of the way.