CASO knows better than I do, but I think the Factory Assembly Manuals shows "non-hardening" seam sealer (which is available in a calk-gun tube); and, I think this is best applied over epoxy-primed metal - then seal/prime and paint.

CASO, would you use the "All-Around" product on the inside joints and the "Fast and Firm" product in the exterior joint, between the upper-cab and lower-cab?

Thanks,