how can i best insulate my static caravan?

I have a 28x10 static which we live in now. As we are going to be there in the winters, I would like advice on how best to add insulation.
1. to externally use boards or rolls of insulation materials, with battens and then waterproof with ? [cladding] - both walls and roof.
2. strip existing insulation and replace with something better [what] and replace panels. how tricky is this?
3. line walls internally - though dont want to decrease internal space too much
We have a gas fire, can replace with a burner though.
I have added thermal liners to curtains.
I have carpets and extra mats/rugs on the floor.
I will be putting a 'skirt' between the base and the ground.
I will consider putting in some/all double glazing if its worthwhile.
Any advice is worth it, incuding possible pricing estimate and ease of the work involved.