[DutchmenSport]

Well, OK. I would suggest then, fill the water tank until water starts coming out of somewhere? It has to have a vent somewhere otherwise you'd not be able to fill it. If they missed the vent hose, water should start coming out from under the trailer somewhere. Keep an eye on everything close, just in case it does overflow inside the underbelly and then turn your water source off quickly. If it does that, you know the manufacturers did not run the vent hose correctly. If water starts pouring off the roof, you'll know the vent went up.

That's what I'd do, even if it means things getting a little wet (just to clarify: a little wet). Again, first sign of water anywhere except through an outlet on the outside of the trailer, kill the water immediately. Then figure it out.

Good luck. I've never heard of anyone not having a vent somewhere?

Another thought:
Have you found the fresh water tank drain valve under the camper? If so, the vent should be somewhere in that area, if your tank is located up front, in the middle, or all the way in the back, it will give you a general idea where to watch for water.