Those Dometics temperature gauge don't very accurately reflect the temperature of the cooler too well, it varies a ton depending on location in the box and I think their temp sensor is mounted on the coldest point against the refrigerator coils. They let in a lot of heat through the top so things up against the lid or on the "fruit shelf" will never be as cold as what's in the bottom of the cooler. Remember its not a commercial chest freezer. Like somebody said before, try to pre-freeze items or it will take all night with the compressor running almost full time to suck all the energy out of those water bottles; and that's only if you don't open or close it and put it in a temperature neutral area where it's not fighting with high ambient temps.
As for the temp you set, unless you're only worried about items on bottom right up against the cooling tubes I'd drop the temperature setting a bit. In actual I use 30 degrees for refrigeration, and 16 if I want to be a freezer. Things that need to be coldest on the bottom, general items in the middle, the fruit shelf for... fruit. Or chocolate, butter, eggs I just want to keep cool.
Remember there are no cooling coils around that shelf. Only what air drifts over from the cooling compartment, and every time you open the cooler most of that air will be swapped with ambient temp air.
Also if you're on 12V make sure you have direct power to the battery and not the vehicle's crap cigarette port wiring. It may sound like the compressor is on 100% but it may be constantly cycling on and off because the auto-cut out is detecting too low of voltage. So you're sucking lots of power but not not really getting efficient cooling. It's good to test the cooler on 110 just to be sure the problem is in the cooler and not the vehicle wiring.