It all depends on how long you have the fire on for, at what temperature, and the outside ambient temperature.
The gas fire , if it is a Truma 3002 is rated at 3.4kw heat output.
The Ultraheat(Electric) is rated at 2kw heat output.
The gas fire uses 240 grams of gas per hour at full heat.
The water heater is also dependant on use, if left on all the time, or switched off between needing hot water, and the fridge of course.
The fridge will use 125w per hour of electric, and about 300 grams of gas in 24 hours
With all caravan appliances gas will heat them up quicker than electric (or cool them quicker in relation to the fridge)
It now depends on the unit cost of the electric and the cost of the cylinder of gas.