Any ladder or storage rack has possibilty to leak, I always double check and double seal the mounting areas. Plus along seam to any rooftop a/c unit.
One old school method is to use a big shop vac to pressurize the interior...switching the hose to it blows air IN not out. Then get up on the roof with a soap and water mix and spray it around suspected leaking areas. If air bubbles apear you've got a leak. To pressurize put and seal plastic over a small window or door inserting the shop vac through it and sealing with duct tape; be sure not to get duct tape on your exterior finish... it's a bear to remove. Allow 5 minutes for pressurization. Or, go to Camping World and pay their $150 to do this.