Remove the cracked cover, place a plastic dishpan over the vent with a brick on it or as suggested install the MAXXAIR vent covers to shield the vents for feature protection. Take the cover with you to your local RV dealer / camping world, they have replacements. While you are at it check your round vent covers (white) that are over the kitchen, bath, vents they may need replacing also... The crank up square vent covers are prone to OZONE damage and are not UV protected. Make sure you get white MAXXAIR covers. The black or brown covers look cool but trap a lot of heat because of the dark color.
Good luck on your adventure of replacing the covers
Sid, Lois & HRH MISSY Terrible 2s SHIH TZU
2011 ROCKWOOD 8285 WS
2014 F-250 KING RANCH PS/SD/6R140/BFT
BOLIY PRO 3600SI GEN
DAYS CAMPED 2016 57 DAYS