You can get heavy-duty drawer slides that are rated to 500 pounds and probably greater. The ones I've seen - the heavy duty ones - are available in stainless steel. I'm not sure what the bearings are though. They're metal, but I'm not sure if they are stainless.

The drawer type slides attach through the faces. I've seen TV drawer slides that have a 90 degree backet that would allow you to attach them to the top or bottom.
The part I can't get my head around is how to design the seals. I keep going with an inner flange design, almost like a drawer face, that would seal around the inside wall of the trailer. The problem is that I can't resolve how to have lower part of the flange and have it clear the supporting structure of the drawer.
Sure wish we could see some more details of this.
BTW - the idea that keeps coming to me is not a slide out to the side, but one that extends out over the tongue when camping.
Keep the ideas coming GPW.
Take care,
Tom