The source of leaks can be hard to find.
You could take it to an RV service shop and have your RV leak tested. There is a video on the following link that shows a leak test.
You could also create your own leak test. If you have roof van that has a reverse switch (draws air in) it might create enough interior pressure when all the windows and doors are closed. Spray a soapy solution around where you suspect a leak on the exterior and watch for bubbles to form.
You'd have to be careful not to cause damage to the fan.
Maybe have the dash vents blowing air in also.
I've read about various methods of make a "home made" leak test. Someone used a leaf blower.