If you ever sleep or cook in your vehicle you want to avoid any material that may hold moisture i.e. fiberglass insulation, the denim you mentioned, etc. We give off a significant amount of moisture just through breathing but also when sweating and cooking releases even more. Cooking in your vehicle is less than ideal but if you travel a lot you will eventually end up doing it, usually due to bad weather. In the summer this is not as much of an issue as the outside heat will dry things but in the winter it can be a real problem. Most interior panels come from the factory with little protection against moisture and rust can be a problem.
With my VW camper I stripped out the interior, treated any rust and addressed any sealing that attention. A light coating of paint to protect the metal then use an audio dampening material applied as per whatever the instructions say. While you can use some of the other materials out there, they may not hold as well to vertical surfaces during summer heat and when bouncing down a road. The various audio forums can give better reviews of various materials that people use and how well they work. You don't have to cover every square inch to get good noise reduction; from everything I've read there is a diminishing return after about 25% coverage. You want to make sure you get any flat surfaces that may vibrate or resonate a metallic sound, but it's not as important to cram it in every nook and cranny supposedly. You can spray a sound deadener or rubberized undercoating over that but beware, most of that stuff stinks and even after some time has passed you may smell it when it's really hot.
Most of the VW people use Reflectix or something smilar, the Mylar covered bubble wrap stuff. It's waterproof and works decent. Thin sheets of closed cell foam works as well. Make sure you get under your headliner. A vapor barrier isn't needed since the material doesn't hold water and it may actually end up trapping moisture which you don't want. Your largest heat losses are going to be through your glass; covering them with insulated curtains or heat shields made of something like Reflectix helps.
