One thing what comes to mind is how much kgs can you put on your tow ball ?
Do you need two bottles ??
A bit of onfo:
A Calor Gas CalorLight 6kg Gas Cylinder is made from lightweight steel,
the CalorLite propane cylinder is 49.7% lighter than the standard Calor 6kg cylinder and includes the Gas Trac gauge
Approx. Total Weight: 10.52kg
Approx. Content Weight: 6kg
Approx. Tare Weight: 4.52kg
Approx. Height: 487mm
Approx. Diameter: 246mm
The 3.9kg cylinder that you allready have will weigh in the region of 9.6 kg when full
The photo's below are of my calor light bottle,
Most bottles have there tare weight stamped on the plate near to the outlet valve
This bottle is stamped with it on the base ring.
So in here when full is 6kg of propane & it will weigh approx 10.42kg
When it's all used it will weigh 4.42kg
But i think you really need to read
this about using propane safely
If you have a look
here it will give you some idea of sizes & prices ( we have a 13kg bottle ) which is cheaper to refill than the 6kg light & i get twice as much gas
& the 13kg is only as heavy as two normal 6kg bottles @ around 30kg...........
but two 6kg calor light bottles only weigh when both are full About 21kg............