Water heater

Hello, we are fairly new to caravanning and need a bit of advice.  We are currently away in our van and have discovered that the water heater isn't working.  We are currently running it on gas and have booked our van in to be repaired.  

Is it costly to use your own gas supply to heat the water? Also, if you heat your water using electricity do you leave the switch on constantly? 

Thanks in advance for any help offered.