The older campers (80's) used a different roof vent cap. I have never bought one of them, but we do have other forum members who have.
The vent is normally 2 parts. A "top lid" and then a "base" that screws to the roof of the camper and is sealed to the roof. You you need both parts or just the top lid?
The vents can often be listed by the camper "opening" in the roof (the base part inside size) and not the size of the actual lid outside.
A club member named Thomascine did replace her 80's camper fridge vent and was successful with it. I cannot find that thread for some reason, she may not of made one or it was part of another thread. But they do make them.
Is your plastic or metal? They had both at one time.
This link to Etrailer who sells some of the aftermarket vent covers may help. I have bought from them and they are a reputable firm. They also have very good details with pictures and dimensions that you may find one here. (just click on the part and they have the details there) If none of these are close, give us some more details on the size and even pics and we can help look too.
Hope this helps, and if and when you do find one that works, please post the brand, part number and where you found it. This helps others in the future. We have a good number of 80's Sunlines coming back to camping again which is great!
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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