Outside vent cover replacement

I need to replace the louvered plastic vent cover located on the outside of the trailer over the water heater panel on my recently purchased 3023. The plastic vent slats were so brittle that they basically disintegrated, leaving an opening to the vent hole. Does anyone know what appliance this vent is for and has someone found a vent cover at Lowes, etc to replace this item without having to go thru TM? The cover is about 4" x 4" square (exhaust hole is slightly smaller).