GuitarPhotog wrote:The only real issue with no insulation is condensation. If your walls/ceiling are the least bit thermally conductive, they will get wet from condensation whenever the outside temp is below the dew point.
I second that, but your 3/4" ply shouldn't be anywhere near as bad as the sheet aluminum I had skinning my last camper. It rained inside at night!
You could probably do pretty well by covering the interior surface of the plywood with some kind of vapor barrier, like a sheet plastic or tyvek. Space it off 1/4" or more for an air gap between the ply and the plastic.
The best solution though is the foam sheet insulation you can find at home improvement stores. It's cheap and a few sheets should be plenty for you. $60 worth of this stuff will make your little camper far more cozy, dry, and better insulated to the noisy neighbors partying next door. Fill the gaps in the insulation with expanding foam.