Hi
Depending on the ambient temperatures, generally you can choose to use either commercial Butane or commercial Propane gas in your caravan, but always check the caravans gas data plates or specification to ensure you have the correct working gas pressure regulators for each gas. Most recent UK caravans have a common 30mBar working pressure for both gasses, but older caravans may require a 28mBar regulator for Butane, and 37mBar regulator for Propane.
Both Butane and Propane are members of the Hydrocarbon petroleum gasses. Pure Butane's formula is C₄H₁₀ and Propane's is C3 H8. But commercial bottled gas is not 100% pure and will contain small percentages of other gasses from the Hydrocarbon series so the specific mentioned below may be be even less noticeable.
Both gasses have some very similar characteristics, and the differences would only really be detectable using scientific equipment, so there is generally no discernible difference as far as the user is concerned, except for the ambient temperatures in which each gas can be used.
The ambient temperature affects the pressure of the gas inside the bottle, and if the bottle is too cold the internal pressure will drop to atmospheric pressure and no gas will be produced.
It's worth adding that when you take gas from the bottle, the liquified gas at the bottom attempts to boil to replenish the vapour you have used. It uses the latent heat of mass of the remaining liquified gas, and in doing so it cools the bottle. The bottle tries to drag heat out of the ambient air around it. Roofing contractors with their frosted bottles of propane gas are a testament to this self cooling effect. Consequently never wrap gas bottles up with thermal insulation, as you will actually make matters worse as the bottles can't grab the heat energy from the ambient air.
The practical minimum ambient temperatures for using each gas are
Butane about 5C
Propane about -40C
In practical terms, Butane should only be used in ambient temperatures above 5C, But Propane will continue to provide gas throughout the whole of the winter in the UK.
The Polisher has written "We are on propane, it also burns hotter so we find we use less gas than on butane." Propane burns with a flame temperature 1930C and with and calorific value of 11,080kcal/kg compared to Butane which has a flame temperature of only 1900C and calorific value of 10.93kcal/kg But Propane is less dense than Butane so a greater volume of propane has to pass to achieve the same mass as the butane throughput So whilst Propane may have the greater heat energy by mass it is less by volume, but whether it is less or more total heat than butane is dependant on the design of the appliance, and the gas supply pressure
To answer RayS last point, I have given the regulator difference at the top of my post. The effect on appliances is that Propane which has a slightly lower gas density finds it easier to pass through the burners injector and emerges at a slightly greater velocity compared to the slightly more dense Butane. But Propane with its lower density has to pass more volume to get the same mass of gas through. This tends to make a Propane flame longer and appear more vigorous than the equivalent butane flame which tends to be more compact but actually produces more heat per unit time.
The calculations to work out the actual heat output differences between Propane and Butane are quite complex and are affected not just by the diameter of the injector hole but by its tubular length and the formations in the gas air venturi mixing tube. But is was generally the case that if Propane was supplied at 37mBar and Butane at 28mBar the performance of the appliance was very similar.
The upshot of the new common 30mBar standards is Propane is supplied at a lower pressure so the appliance will produce less heat output, and Butane is supplied at a slightly higher pressure which will produce more heat compared to Propane. It's worth pointing out that where an appliance has a thermostatically control modulating control valve, once the appliance is up to temperature the thermostat will compensate for any differences in gas type heating effect, and you will not be aware of any difference in operation.
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