Hello Graham,
All the normal caravan (and motor home) appliances utilise vapour take of bottles. This means the valve in the bottle is onel exposed to the vapour above the liquid. The regulators are only designd for vapour. Where as most LPG motive power installatoins use theLiquid take off tanks. You must never use a fridge or any other normal applince with a liquid take off system as the pressure will rise far too high, and you may get liquid gas to the burners rather than the vapour.resulting seroius over gassing and the possibilit of a fireball.
Withe regards teh VAT, In theroy yes you could, but I think the practicalities of proving to the satisfaction of the VAT man the sole use of the fuel is non motive domestic heating may be difficult. They are more likely to say its your choice where to buy the fuel and you chose the high price option.