Hi Al! Of all of the things you can say about
fiberglass RVs in general is that there is no single way to do anything! Dometic has been wonderful over the years at supplying fridges that vary enough from model to model that nothing from one fits another, and no two models mount identically! There are probably as many different sized cut-outs in trailers over the years as there are trailers!
Generally, though, there will be several screws into the cabinet on the "outside" of the
fridge; e.g. facing the cabin, on the flange around the door. They should be pretty easy to find.
Now comes the "fun" part. Outside, you open the vent panel. In the back you'll want to disconnect the 12v lines (if it has them), and then the 120v AC should be plugged into an outlet somewhere behind the fridge, or to the side of the fridge under the sink. Disconnect that. Last, turn the gas valve off (and make sure it's off at the tanks of course) and unscrew the line from the fridge. Make sure you don't damage the tubing, 'cause it's a pain to re-run copper tubing. It can be done, it just isn't any fun.
Now there should also be two or three screws securing the back and bottom plate of the fridge to the platform on which it sits. Remove those, and barring any owner-modifications in the past thirty or so years, your fridge should slide into the cabin.
Good luck!
Roger
You didn't mention what model of refer, but in the Document Center there are a bunch of refer manuals. Dometic Manuals usually have installation instructions...guess you could use that to de-install: 
Linear Mode
