my fridge has a 175 watt element and doesnt work properly on 12v any more than a couple of hours,would a 275 watt do a better job

You will destroy the cooling unit of the fridge if you overheat it.
The issue will not be the element, but, the power supply to the element.
You will be getting voltage drop in the wiring from your tow vehicle to the element.
The wiring is what you will need to attend to.
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