If it's the fridge vent, it has two pieces: a removable cap and a more permanently installed base. The cap is basically disposable, a light encounter with some brush can break it, buy a new one. They are attached with 4 screws. If the base is also broken, it needs to be replaced. Being RV gear, these are c**p, flimsy pieces of poorly injection molded plastic. On the Bigfoots I've owned, they dealt with this by putting a screw every 2" or so around the flange. They would have used butyl sealant under the flange, then spread some horrible brittle stuff around the outside which never does any good, and I'd not replace that once you spend a few hours removing it.
I'd avoid replacing the base at all if it isn't badly broken. If it is, you'll have to do what the factory did, it would be great if you could match all the holes rather than drill 25 new ones but it will be difficult to do. I molded a new base from fiberglass for a more permanent fix when mine broke. Since it is rigid, it only took a few screws, and then only to hold it in place until the polyether sealant cured.
Bigfoot 10.4E, 2015 F350 6.7L DRW 2WD, Autoflex Ultra Air Ride rear suspension, Hellwig Bigwig sway bars front and rear