I have done some checking locally and neither Home Depot nor Lowe's nor any other chain building supply stores carry 5/8" (19/32") plywood. However, one of the few remaining local building material stores, Star Lumber, carries nominal 5/8" exterior plywood in both AC (about $34/sheet) and AB (about $44/sheet) and both are decent looking. It's interesting about what you can and can't get and where.
Although my trailer has 5/8" OSB floors, I'm planning to use 3/4" plywood as it's a better thickness match to the adjacent OSB that is still in fairly "good" shape except for some swelling due to moisture and the fact that it's sitting on some fiberglass insulation. I plan to plane the perimeter that goes into the C channel.