Is it a high top van? Any windows in the roof? Visually inspect the caulking around the van roof. Check around the fan if you have one.
During a really heavy downpour mine had water get in that I think came in around the furnace vent. Instead of re-seating / reinstalling the assembly I took the easy way out and caulked the perimeter with high temperature caulk.
I haven't had any water show up on the floor since.
I think water could get the fridge vents also during a really heavy downpour.
Water leaks can puddle or become visible a long way from the source of the leak.