This can be done in a retrofit but to be fully effective it is the first thing you address prior to any internal construction.
There are several types of insulation and they vary enormously in cost, effectiveness and purpose.
1. Sound deadening and insulation
2. Inside panel insulation (walls and ceiling)
3. Painted on insulation
4. Under floor insulation
Sound deadening insulation is a foam or rubber type sheet that is self adhesive and sticks to the inside of the van walls and ceiling. As its name implies it is designed to reduce noise (especially while traveling), and provide the first layer of insulation.The sheets are normally about 400mm x 400mm in size so are easy enough to manage. They can be cut to shape if needed to fit into all areas with a decent pair of scissors or a sharp knife (Stanley trimmer or similar). They can make a huge difference to the noise quality in the van when traveling as they reduce the vibration of the body panels as you drive. They also reduce the likelihood of condensation on the metal surfaces in humid and cold destinations.
Insulation in the void between the panel walls and your internal panel work can be with any number of materials including cardboard or shredded paper, glass fiber sheets or batts, foam or polystyrene, or insulation rolls. This is an effective area to insulate as long as you make sure you fill all the possible voids. The ceiling, walls and underfloor areas can all be done with sheets. Try to ensure the insulation in the wall cavity can’t just slump down or collapse as you install it, or later on when traveling.
Painted on Insulation is usually available for the roof area of your camper, or caravan. There are a range op products and despite all the claims I don’t think one is better than any of the others. I personally prefer the water based acrylic type of roll on (or paint on) as they tend to adhere better and seal with a small amount of give when operational.
I also prefer the bright white as a colour as it is more likely to reflect heat and light to a greater extent.
Under floor insulation is possibly the hardest to retrofit but anything you can do is better than nothing. If the floor has a space beneath it then adding a layer of insulation is great. Some extra things to consider could be to spray the underside with a bituminous or rubber based paint, or to add a carpet (or insulated vinyl) floor covering inside. Anything is better than nothing.