You can do it!
I just replaced mine a couple of weeks ago because I could no longer close it tight enough to seal, and had wired it shut several years ago. I used the Fantastic vent, and found it very easy to modify the opening to fit accurately, using left-hand aviation snips. If you do a search under 'fantastic vents' and you can find plenty of information and pictures.
I removed all of the OEM fixture on the inside, then on the roof used a sharp putty knife and hammer to shear off the rivets between the aluminum skin and the old vent frame so I could remove it. After cleaning off the old vulkem and squaring up the corners with the snips I prepared to install the new vent on top according to the directions. Some of the screws they provide stripped out, so I removed those in particular and replaced them with olympic rivets. I covered all of the screw and rivet heads with a generous layer of vulkem, tapered toward the outside edges with a dry putty knife.
After finishing on the roof, I came inside and layed out where I needed to cut so I could line up the inside trim, and cut using the same snips I had used on the roof.
I replaced the ceiling lights with a standard fixture that I had extra when I replaced the oem fixtures with Thinlites. I mounted it just ahead of the vent opening, easily redirecting the wiring I needed. It is still turned off/on from the switch just inside the door, though it isn't as bright as all the bulbs in the OEM fixture, but looks like it had all always been there from date of manufacture. I like the results.
Good luck with your decision!
1996 28ft Classic
2008 Silverado Duramax 2500HD