Hi Stathopoulos,
Congratulations on the new rig and welcome to the Heartland Owners Forum.
If the 18 is a gas absorption frig that runs off either 110V AC or Propane, that's great for boon docking. If the 22 is a residential frig that runs on 110V AC only, that's great if you're always going to be on shore power.
Residential refrigerators (like the refrigerator in your sticks and bricks residence), are larger and cool better. When towing, they run off the trailer's batteries. But if you plan on camping without shore power for more than an overnight stay, you'd need a generator to keep from running the batteries down. That's where the gas absorption frig shines because it use only a small amount of 12V DC power along with propane.