With the price of gas and electricity so variable in the context of caravanning the breakeven point will vary quite a lot, but I think in most cases the smaller the gas cylinder the more expensive the content is per kWh and is more likely to be be more expensive than EHU power.
Whilst unit pricing may be a concern for some, there are other factors that might swing the decision about whether to use gas or not. Families may need more heating especially with young children so the limitations of electric only might need to be supplemented with gas, but on the other hand the convenience of an EHU and not having to bother with buying and changing cylinders and the weight penalty they bring could mean EHU only swings the decision. Are replacement cylinder available in the area, with the difficulties Calor and their customers have been reporting, there could be supply problems to consider.
There is no universal right or wrong, it's about what compromises you're prepared to put up with.