If you do a search here, you'll find the name of at least two sites that tell you the original fridge model, it's dimensions, and what you may replace it with. When I had mine done, the space was limited by the floor, the left interior wall (where the extinguisher is), the right pantry wall, and the upper space level with the pantry "roof". It was framed just like I see in homes, with space around. They screwed everything in and made it real tight. Since mine was also not original, they had to use a few extra screws in strategic places, but no alteration in the fridge space. It fit snugly under the original vent. Although mine was built in '81 on a '79 P30 chassis, I hope that your Argosy may be close enough to my design for this to help.
1992 Airstream 300LE, rear queen, Banks Power!