[Diesel-Gypsy]

The under-belly coroplast is far from sealed-up.
Driving in the rain can cause water to get up into the insulation that insulates the under floor area.
Once there is water up in there, you never actually see it, but after a while you can smell it.
The 1st. time I dropped the under-belly liner I was surprised to see the wet insulation, and the source of a mouldy odor.
I pulled out all the wet / damp insulation, replaced it with new, dry insulation, then tried to seal-up the under-belly liner.
I used caulking & black duct tape.
No more smell.

Check your 2 rear storage compartments as well because they can get damp inside depending on various situations.

Rick