Highway, Nice job so far on your box, should make for a good solid rig. I would think that spray foam insulation in the cavity formed by the metal studs and the wood rub rails would be the way to go. The foam will get in behind all the spaces and bond to everything reducing a lot of road noise and creating a good thermal barrier. Just remember to plan ahead with any wiring or plumbing as adding it into the wall later is more than a pain. one disadvantage with adding the foam after siding is there can be quite a bit of heat from the chemical reaction of the foam setting up and can distort the siding if not careful. However the advantages of not having an insulation material that will absorb moisture within the wall cavity is a big plus.
Dave
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