Parts search comes up with 5/8 only, but.. From the original print, it looks like there is a 3/16 lauan board first, then set back (for the aluminum) is another 1/8 topper. Jayco could have beefed it up on the line, while under production for a heavier duty roof. Or.. could be the interior lauan board for the ceiling.. That would make more sense..
After my ramblings.. it should be only 1 layer of wood. I would go with 5/8 for strength, but you can go 3/8 for ease of work, depending on the distance of the supports.