For pickup bed storage, "Highway Products" boxes I found to be the best. A long time RV'er and friend recommended them to me. He had the large bed-box, and said "one could easily put four bags of golf clubs in there"! lol
After considerable looking, I also bought the bedrail level storage box. Yes, it is fairly expensive, but it is extremely well made, and has a very good locking mechanism. I only carried one bag of golf clubs in it, but I also put some of our folding chairs in it as well., and a "lot" of other various stuff.
Depending upon how much additional storage you need, Highway Products also has a tailgate storage box. Below is a website for your review. Their boxes are truck brand specific, for fit reasons.
http://www.800toolbox.com/5thwheel/5thwheel.htm
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