Cool build!
As you likely know by now, there is only so much need for strength in the adhesive. Quickly, the next "layer" of foam delaminates from itself, either due to compression, torsional/flexural, shearing or tension-based stresses. This is where structural foams such as Diab's Divinycell and plastic or alloy-based honeycombs, such as those made by Plascore, come in. The strength-to-weight ratio is quite impressive, allowing structural sandwich panels to replace tube based structures in many applications. For a small camper, where you do not need ultimate strength, this is less of a concern. Sandwich panels usually fail because the foam core is weak in shear. As XPS is insulation, the strength isn't great, but with the cage you have supporting it, I would not expect any issues other than some pretty serious cold bridging/condensation issues.
