Ok I'm going to have put up a square tubing garage 25'x23' for tractor & SXS parking and other farm stuff. There in the shop now and out of the weather but I need to open up the shop some more that the only reason I'm doing the extra building. I should have made the shop 30x60 to begin with but that water under the bridge now.
Building is going to be around $10k and to keep costs in check without pouring concrete I was thinking about laying down pond liner which is 45ml then using river rock over that since it's smooth and less chance of punching the liner as a moisture barrier. As for driving on I'd use 2x12 planks on top of the liner before laying down the gravel.

Any better ideas to skin this cat?

Make it bigger. Put insulation on the inside of the roof, so condensation won’t drip. Run water and power to it. Add a half bath (toilet and a sink) and septic. Better to go with the concrete. And add a workbench in one corner.

No matter how big your barn/structure is, it will soon be too small. A fellow told me that and I laughed. He was right.

All you mentioned is in the shop. The is just for parking/storage without the moisture.

As I mentioned, my barn shrunk over time. I had concrete poured (20x20’) on both ends and had each roofed. And I filled those up. For the travel trailer, and for the RV I had big carports (for lack of a better name) built, and they have gravel floors. The gravel works fine.

IMHO, river rock is not the best choice here. It does not lock together.. Pulverized granite will pack into a concrete like surface. Crushed limestone or granite would me my next choice, put as you mention, may puncture your membrane. Is you are so worried about moisture that you need the membrane, then you really need to be doing concrete.

you just need limestone base rock pad, no moisture barrier is necessary. Put a layer of 3 inch covered by a layer of 2 inch. You can put a cover gravel over your pad after you get it set up good.

Completely enclosed structure (4 walls w garage doors)? If so, why not just do a 3 sided "tractor" barn. Shouldn't have to worry about moisture, probably cheaper as well. I have a 30x60 enclosed shop and 30x80 3 sided tractor barn. Both are full.

agree, river rock is loose and will have ruts where weight gets driven on/thru. I used river rock in an corner area of our driveway where we had to swing wide when towing a trailer, the rock shifts and ruts when you drive thru it.

I am not sure of the technical term for what I grew up calling chat or trailing's from a rock crusher. It is what is last thing when going thru the screening process, usually the cheapest material to buy also. After it sets up and gets wet a few times, it sets up very good to traffic and doesn't track to shoes or tires. I brought in about 20 yards into our back yard to level out an area to put an above ground pool on, then put about 1/2" to 1" of sand on top to prevent any possible punctures to the pool liner.

as others have stated, build bigger than you need as you will quickly outgrow the space.

How about adding a lean-to on the current shop?