I also find normal spice bottles seal acceptably well to generally keep clumping at a minimum, at least with New England weather.
For salt, the usual trick is to put several grains of rice in the salt shaker, and it works quite well. Undoubtedly other things besides rice would work equally as well as a desiccant, but rice has the advantages of being non-toxic, safe for food use, and readily available inexpensively.
If you're keeping non-perishables in the camper, and even if not, I strongly recommend also having some good mousetraps and checking them fairly frequently.