LPG

my understanding is that the max that can be carried is 13kg and that comes with certain requirements best check it all out first john

You can't use the exact tanks used on car engine conversions. Engine conversions require liquid take off (from the bottom of the tank). LPG for cooking, etc inside a camper requires gas take off (from the top of the tank).
So the tanks are different, and can't be used.

However, there are tanks that can be used.

The simplest and cheapest refillable system is Gas Low. A normal looking LPG tanks that can be refilled
http://www.gaslow.co.uk/

And also bigger tanks are available to strap underneath the chassis
http://www.gasit.co.uk/cubecart/index.php?act=viewProd&productId=18

Yes actually you can use the LPG tank from a car conversion, they are not different tanks from ones used for cooking/heating they are exactly the same but the fitted hardware is slightly different.
Car conversions take the lpg from the bottom of the tank as liqid and for the cooking/heating its taken as a gas from the top of the tank, therefore you need to adapt the pick up pipe from picking up at the bottom to picking up at the top of the tank, making sure the 80% filler shut off still works properly and then everything needs to be tested by an authorised and appropriate gas fitter.