There's a ton of videos on YouTube about how to replace the lids on RV fan covers. As LJAZ mentioned, you'll need the brand/model to match if you are just replacing a cracked cover. Probably worth replacing the hole thing while you're up there, in which case you only have to match the opening size!
Look for fan covers or vent covers. NOT Skylights.
These aren't skylights, they're vents with fans and covers. Big difference if you are searching for parts and How To videos.
2012 FF 214WSD and Equalizer WDH
2019 GMC Sierra SLT, Crew Cab, 4x4