replacement for rotted wood

i have an older trailmanor 1993 2720 and i am in need of some replacement parts.the part that i need is the long piece of wood that is under the weatherstrip panel on the rear/right side of the camper.i just bought it used and i noticed this piece of wood is rotted out.is there a replacement piece that i could order from a replacement parts website?i need this to make the camper more weathertight and keep out drafts