Lisa,
First we need to know if your existing unit is specifically for butane, if so its likely to be maker as such including a pressure of "28mbar".
If that's the case its easier to go to a camping/caravan/motorhome accessory shop and ask for a propane one which will almost certainly be red and marked for propane and be 37mbar pressure.
You will also need half a metre of rubber gas pipe plus a couple of "jubilee clips", get the pipe from the same dealer; the orange one being the wiser choice.
If its a modern van or one with a 30mbar regulator then that regulator is suitable for both butane and propane and you need only a special propane pigtail pipe to join it to the propane bottle. Its essential to buy a made up pipe in this case.
There could be other complication if its a continental van, if so ask again please with more info.
John