On the tailgate of my new trailer, the guy who built it used 1/2 inch square tubing to reinforce it. Unfortunately this didn't leave the best surface for cooking and such. I was thinking of using some kind of thin wood as a table top that I could permanently mount to the tailgate but I don't know if there is anything available that would hold up to water and general outdoor use.
Can you still get teak? I know they use this a lot on boats. What about good sealing products?
You can still get teak, but it's expensive. If you nose around boatyards you might find some scraps for cheap though. You can also look around at thrift stores and find old cutting boards, which are sometimes teak. Collect a few of those, cut them up to the right shape and size and voila, a good cooking surface/cutting board all in one!
You don't need to use Tung oil with teak either. Sailors in the south pacific would use limes. Yep, that's right, limes. They're what was available and they have a natural oil in them that protects the wood. However, a lot of sailors never treat their teak and prefer to let it turn grey, but that's because it has more grip for walking on.
Another good option might be finding a hardwood plywood and cutting some thin strips of solid wood to cover the edges of the plywood which is a weak point where water will enter the grain and ruin the wood. You could always cover this with a water-based polyurethane. But, then you couldn't use it as a cutting board.