They don't want to be liable for giving figures that might or might not apply to your situation. And to be fair, they may not know. The vast majority of 12v fridges are generic. Then stamped and labeled to the brand. My current 12v fridge model is used by no less than 5 brands, including Alpicool. The difference being that the better brands so things better regarding that fridge. Customer service, better packaging sometimes, better warranty, etc. It's usually worth the small bump in price.
I would do research on the 12v fridges account to you, then read reviews. Pay attention to customer service, and reliability. Overall, the ones that have better reviews tend to use about the same amount of energy given the same size and conditions.
Plan on the fridge compressor running 30% of the time, and you'll probably be right on the ballpark. 40% if you want to be doubly certain. If you calculate for that, you should be ahead of the game.