without seeing the job i would use a 60 or 100amp henly block from your existing incoming supply.t would be a good time to upgrade your board tails to 25mm and 16mm earth unless already had upgrade.i would not use the home board and ibstall a small garage board with a 60amp main switch and a 40 amp rcbo to the caravan or a 40amp cb if the caravan board has its own rcd.6mm supply would be ample for small to medium sized caravans.had varied results with earthing rods so would prefer to run a seperate 10mm earth along side the 16mm armoured.if glanding off is a problem in either board then a 4mm x4mm ip44 or above plastic or galv box would make life easier