A quick search on the 'ol web and I found this: https://www.canvasetc.com/product-category/canvas-fabric-cotton-duck/mil-spec/

Military grade as per original OD Green canvas. Is this what you're looking for?

The waxed canvas might do the trick, since it's water repellent. Just wondering how a cotton fabric will stand up to UV rays over time...

Just know this. It stinks like an army truck if you've ever been in one. LOL I've used the same stuff in some P 51 Mustangs and Sea Fury's (old WWII airplanes) in the past. Tough as nails and not a "pretty " cover by all means, just functional as the Army want it.

The newer military trucks use cordura, it has a water resistant coating on the underside.

I use StaSof #10 duck OD green. Softer Feel, easier to work with and looks relatively the same. Has a dry finish waterproofing. doesnt stink and you don't get the wax finish all over everything. Available from trivantage