Any competent welding shop could make you a box, if you're not skilled in making your own.
But, if you've got basic shop skills, I'm with PartyOfFive (do I call you "Five" or "Party" for short?). Aluminum is easy to work, comes in lots of useful shapes, and can be found in big boxy stores (Home Desperate and the like), so it's easily available pretty much everywhere. Note that you may pay more at a big boxy store than at a commercial metal supplier; shop around for price.
But think through what you want in there before you make/commission a box. If you're putting in a battery, you'll want the space vented, and you'll want to choose a material that can handle any battery leaks. And you'll want to integrate battery tie-downs into the design, rather than awkwardly adding them later. If you're going to use it for general storage, you may need to weatherstrip it to keep water and road dirt out, which will require a more elaborate door. And so on.
Have you concluded that, given your travel habits, you'll never need a generator? We don't use ours very often, but it's been right handy the few times we've used it. I do wish Itasca had chosen a Honda, though; the Onan that came with our rig is a touch noisy.
