The door to nowhere: questions

Some folks use that space for an extra battery and things that don't need to stay clean and dry.

On the outside of my rear door, I fastened a plastic storage bin, over the hole. I use that space to store small lite weight items. The bin keeps them clean and dry.

Don't step through the door...

You're correct that anything in there will be subject to whatever weather you go through on the road, but that still leaves a lot of things that can be stored there. One use I make is to store dirty laundry stuffed in a garbage bag.

Spare Beer... wait - that's an oxymoron

Laundry in garbage bags, extra 24 of coke or beer, battery, Its good usable space for things that can weather the weather.

Last year I helped a grandma in distress, who was camping with her grandson. Her husband left earlier to go to work, when the little guy locked the truck keys in the truck. She also left her cell phone in the truck. She was beside herself to say the least. I tried to help, and then noticed the forward access door to the pickup bed. She reached into the space and found the spare keys. I've never seen anyone that relieved. (The camper almost touched the bed rail so there was no access from the outside)

It's a good place for fishing gear: hipboots, tackle box, small net, pole if it breaks down into a metal tube, etc.

Bill

We load the space in front of the wheel wells with full plastic drinking water bottles, usually about 24-30 using both sides. When the bottle is empty, we throw it farther forward in the compartment for later recycling.
jefe

I keep a short handled shovel in there. Never know when you will need one.

i store a shovel,axe, fire poking stick and my metal weenie roasting sticks there.