Thanks for all of the great ideas. A couple of them were just what I was thinking about, so I'll investigate further. Another possibility, although not removable, would be to build a custom box on a flatbed pickup.
My concern was more about stuff blowing around while we are away from the campsite than theft. Yes, it could all go back into the truck when not in use, but that would be added weight that we're hauling around while sight-seeing. Partially offsetting that, though, is the added weight of the removable box that we would be hauling when moving from one campground to another.
Much to think about.
David Lininger, kb0zke
Heartland mpg 181 (sold)
1993 Foretravel U300 (for sale)