Originally Posted by Diplomat Don
From what you're describing, it sounds like the wheels are kicking water up high into the wheel well and it's draining into that box.
Yes, I agree.
I've been trying to eliminate a similar leak on my motorhome in the bay in front of the driver side front wheel, and finally tracked it to water being thrown up by the wheel with enough force to go through a seam and run down the inside of the front bin.
You could tape some newspaper to the inside of the bin and go for a drive in the rain to see if the water is coming in from the top of the bin.