I found one of my trailer's wooden planks is rotten just below what looks to be a good board. If I'm to replace one board, I'll replace them all. My question is what kind of lumber should I use? Any pretreating of the wood prior to installation? This trailer sits out in the weather and sun with no protection. I was using used oil on the boards for protection, but it seems like that's not currently going to do anything for these boards. I didn't do this for the first 4-5 years I owned it. I'm not sure what kind of wood was used by the factory, I'm not a wood expert by any means.
My trailer is 20' Load Trail with full length boards.
Any suggestions that won't cost me thousands of dollars?