I did something similar to this pic,
On our previous camper i installed 2 short (12"or so) shelf standards to the outside wall. I sealed around the screws with eternabond. The brackets slip in/out very quick and easy, making the counter hieght adjustable to different campsites. I used a laminated pine board around 18"x48" that stored on one of the bunks while traveling. It was under the awning when in use, so exposure was not an issue. You could cut a sink into it and place a bucket under the drain. We used this setup for a few years before we bought a new camper, the factory finish(white painted) on the standards held up very well. I got them at home depot or lowes.