Water in wet bay

We have a 2016 Tiffin 36LA. On a recent trip when I stopped for the night at a campground and opened the wet bay to hook-up the water and sewer the bottom of the bay was full of water. It was clean water so I ruled out the black tank leaking.
Checked the water pump it was not on.
Checked the hose inside the bay - was turned off and shower head also off.
Both tanks were completely empty - had dumped prior to leaving the previous stop.
We did go through very bad weather - pulled over three different times because of extreme rain. Is it possible for road water to get inside the bay?
The next three days of travel no problems. No bad weather either.
Anyone experienced this before?

After reading some of the responses
I should have mentioned it was about 3 gallons of water.
My wet bay is just in front of the rear tires.
My bay has the portal at the bottom that screws on and off. It was sealed because the water was full in the bottom of the bay.

The water was not drinking water clean. There was some dirt. I should have said it was not like grey water dirty and definitely not sewer water.

I will try the hose water on the outside of the bay to see if it is sealed. Thanks for your help.