I can't say I've heard of something that large before. It might not be original.
Yes, finding a replacement will be difficult. Usually these days the size is pretty much the same, just the hinge differs.
One thing comes to mind though. I thought they were larger than this, but check out truck camper roof escape hatches. They are the type of things they usually put over the cabover of a truck camper so that someone could get out if they had to. They were larger than a normal roof vent, but I don't recall how large. Perhaps that is...the RV dealers won't have them, but you'd have to either special order it or find it online.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR