The manufacturer of the vent cover is Ventline. There are other manufacturers of these vents/covers, specifically Jensen and Elixir, but Ventline is what Sunline used from the factory. There were a couple different sizes of vents, but the larger ones were mainly used as escape hatches on truck campers. Like Pat said, most RV dealers have this standard size and might not carry any other size. Regardless, I don't believe there were any sizes close to this one. The best foolproof way of knowing you purchased the right cover is to take the old one with you. Just make sure you don't take it off when it's supposed to rain!
The cover is quite easy to change. Here is a step by step plan of attack:
1. Open the vent from the inside.
2. Put a ladder next to your coach and lay something on the roof if you can't reach the vent. If you can just reach over and put minimal weight on the roof, then you don't need support.
3. Remove the clip holding the lift mechanism to the cover. If you cannot do this from the roof, you can do it by way of the inside, with the screen removed. To remove the screen, remove the 4 screws holding the trim cover on, then remove the screw holding the crank on (and the crank), then remove the two screws holding the screen on. These are way up on two sides.
4. Take a pair of needle nose pliers and bend the little tab on the end out. The cover is mounted on a C channel, so you need to "open up the C".
5. Once the tab is bent past where the rest of the hinge is, just tip the cover forward all the way and slide it out the opposite side from you. Again, this step may be easier to do from the inside, though you will have to get back on the ladder to get the cover itself.
6. Slide the new cover on the channel, starting with the end that is not already bent. Once the cover is all the way on, bend the tab in like the old one was.
7. Reattach the crank mechanism using the clip. I like to put a little white lithium grease on the track at this point.
8. Reinstall the interior screens, etc., if you gained access to the cover this way.
9. Test the cover to make sure it seals properly.
10. Come back here to SunlineClub if you have any other questions along the way.
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