I've had lots of water in the underbelly, but mostly from defective plumbing inside the trailer. I saw the trailer in the dealer's service bay with the underbelly dropped and there doesn't seem to be any sort of sealing where it fixes to the chassis. There's huge amounts of expanded foam around pipes, etc but nothing on the edges or "patches", so I doubt if it's meant to be waterproof from rain and road spray.
Lacrosse 301RLS Touring Edition
GMC Sierra SLT 2500 4X4
BMW F650GS Dakar
Suzuki DR350SE & Suzuki DR650SE
Specialized FSR XC Comp