All Dandies should have an insulated roof, it's basically two layers of PVC with padding in the middle. The walls on winterised / insulated are also padded, double skin, ones that aren't have one layer or single skin of PVC for the walls.
The double skinned walls are usually, but not always white on the inside, single skin walls are the same colour inside and out, you can tell the difference by looking and feeling them.
Some also have an underfloor gas heater fitted, but if you're using electric hookup, you can use an electric heater to save paying for gas and electric. Winterised usually means having the insulated walls and underfloor heater, because this is desirable it also pushes the price up a bit.
Mike has made his own insulation, and we're in the process of doing so, although we've camped all year round to date without any. Ours is more for when we haven't electric - we do a lot of classic car shows and steam rallies.
Is that as clear as mud, as Mr Itch would say, or does that make any sense and actually help?