By "tin" I assume you mean sheet metal (steel). Tin is not magnetic and a magnet will not stick to it.
Since the plastic "face" of the Dometic fridge (not the stainless steel fridge) is designed to be removed so decorative wood panels can be used, the black plastic sheet can be removed and replaced with powder coated or painted sheet metal as SeaDog suggested.
The advantage of removing the plastic rather than gluing a piece of sheet metal to it will allow you to return the fridge to OEM if you ever decide to upgrade the FR3.
The Stainless Steel fridge is a problem because stainless is also "not magnetic" (except for one formulated with a large percentage of Chromium).
Here is an article on 7 possible "fixes" for fridge magnets that don't stick.
Stainless Steel And Refrigerator Magnets - 7 Solutions | eBay
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