I need some help to trace an unusual smell in our BT

We have a rouge smell in our van that I can’t trace. It’s a strong smell of soap powder & is only present in the cargo hatch & under the bed.

I have completely removed all items & cleaned out both the cargo hatch, & under the bed. When you leave the bed down & the cargo hatch doors closed, the smell returns.

We have our battery pack with the four original batteries under the bed. These batteries are now five years old & are still in good condition, with enough grunt to run the microwave & a 240v TV. The battery box is also in good condition, with the gasket & vent hose as they should be. The batteries, once the thin layer of dust was removed still look like new, with no obvious signs of any leakage. I have leaned over to smell the batteries, & can’t notice any smell. (I’m not that sensitive to smells, but it affects my wife. She cannot use perfumed soap powders in the washing machine)

By the way, we have a washing machine installed behind the ensuite, & soap powders stored on a shelf above the machine at the other end of the van.
We have never stored any soap powder up front, & closely examining all of the bits & pieces that normally live up there, I could not see or smell any evidence of soap.
This is a new problem, which only became noticeable after a short trip last month.
Batteries are the most suspect cause, any ideas?

Neil & Pat