Options for large scale secure storage are limited for sure in campers.
Agree, unless a thief is hurried, they would be more intrigued by something that is not commonly locked that happens to be locked particularly.
This topic came up recently and one suggestion was a "dummy" safe. Cheap or old safe, loosely attached to the structure (to "appear" bolted down), fill it with some rocks and scrap metal, so it sounds and feels like something worth while. And then creatively hide the valuables elsewhere.