Everyone seems to want to tell you to not have cash. They are not answering your question, which is a reasonable one. I would recommend you have something built in to the floor deck somewhere and perhaps disguised under something greasy and oily looking so that no one would want to touch it. This may be visible from underneath so you should particularly disguise it there as well.
I would start by asking yourself if you want to have it easily accessible from the inside of the van. If it doesn’t need to be easily accessible you can perhaps conceal it inside a phony propane tank. Often times vans and pick ups will have propane tanks mounted underneath-long narrow ones. That might be a good place to have a rifle or other firearms as well, provided you could remove an end cap. The seam could be concealed by a strap for example. Magnets could hold it on securely.
if you want it accessible from the inside you could have some sort of a drawer, perhaps by removing an airbag on the passenger side, and Aktivated by a hidden switch. If you look on YouTube you’ll see some clever solutions for that. Whether or not you carry a lot of cash it’s handy to have hidden compartment the store valuable documents such as passports ID, jewelry and watches, or a camera. Good luck and God bless.