Hope any of this helps
Sounds good so far, but when you say ‘hardboard’ do you mean that or thin plywood..? I ask as hardboard tends to attract moisture, goes a bit soft and sags quite often in the ‘harsh’ environment of a camper interior. The wild temperature variations it can experience and condesation from cooking and bodies do it no good at all – plus it lacks any stength to screw anything against.
That point out of the way, a lot of people use thin carpet inside as this adds an extra layer of insulation – just glue it on. For my own campers I just paint them, as they can be wiped down and repainted to suit yourself – plus I find carpeted walls and (especially) ceilings are a bit bizarre and dark in many cases – but each to their own. Household paints are fine on the inside – especially those designed for kitchens and bathrooms. Beware the extra condensation you’ll get on any exposed metal surfaces though.
My camper is a ‘stealth’ too – because it’s just so useful to be able to camp everywhere..! – I have no windows in the rear, but I have cut a small hole in the side over the cooker to provide a vent – covered on the outside by the type of aluminium vent grill you can buy in any DIY shop. These are often seen on the sides of vans, so doesn’t draw any attention – until, that is, the steam from a boiling kettle starts to waft out through it..!
Enjoy your building