Unfortunately, this has been an issue with a number of Arias. Check for foam sealant where the frame rails enter the cargo bays. Water sprays up on the rails and runs down into the bays. On our coach in particular the bay door seals were installed without being pushed up or down into the corners leaving pretty large gaps ( when I found this, Thor sent me enough seal material to redo all the bays). Also on our coach, on one side of the bay with the roller drawer, the opening had been cut to far forward so that the seal, even when installed correctly, did not contact the door and there was a gap of almost an inch for water to enter. This was corrected by adding metal to the opening, placing the seal in the correct location. Also unfortunately, the plywood used in the bottom of the bays pretty much disintegrates after becoming wet. Watch for them to begin sagging below the coach. Thor has been mostly pretty good about correcting the flooring issue, even out of warranty from what I understand.
Vicki and Mike, Aria 3901- The Wined Up Bus, towing 16 Jeep JKUR.