Hi and welcome to the forums!
I don't have a fifth wheel... anymore.
I do have a Class C with not a massive amount of storage. We are currently using the stackable Ziploc weather tight storage containers
. They are water proof and have channels on the top that allow them to lock together to minimize shifting. For some reason they are not listed on the Ziploc website, I have found them in some Walmarts and Lowe's stores. The link is for Amazon to give you an idea what to look for. We have about dozen of them in various sizes, I color code and label the ends of them so I can tell at a glance what is in them.