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Your question asked about *opened* packages.  I'm going to offer a bit of a frame challenge.

Don't store opened packages long-term in your trailer; rotate them into your kitchen and use them, and buy another unopened package for the trailer.  If you're asking about long-term storage then it sounds like these impulse trips are not so frequent that this would be wasteful, particularly if you couple this with buying smaller rather than larger packages.

After you open a package and while it's still in your trailer, I've found that glass containers with good lids (like mason jars) work well.  For shorter periods (a couple weeks), good reclosable bags keep things dry and bug-free on shelves in my trailer.  (We don't have to worry about critters getting in and chewing through them; your mileage may vary.)  I've had bad luck with the bags that have a plastic slider and prefer the ones where you have to press the seam together yourself.