I am using regular non-treated 2x10 pine boards. No sense in using treated, it only keeps the bugs out of the wood. Won't help with weathering.
I went to the local paint supplier and got 2 gallons of Cabot tinted stain.
Exterior paint is about the best for sealing the wood, I didnt want to paint mine a solid color.
One thing you shoud consider is the hardware your using to secure the boards. Stainless was too expensive for me. I got some zinc plated bolts for mine.
Might try calling some Lumber suppliers. Sometimes they carry trailer specific decking. Usually a hard wood like oak. But its expensive.
Bruiser