If you are going to be using the HWOD for as many as you say I'd definitely take the 60ltr.
As for the gas usage, I've been using the same 4.5kg cylinder for cooking and showers for the last 5 or so camping trips and still have plenty of gas, so you wont have any problems with a 9kg.
AliKat wrote:So I think if we use the HWOD plugged into the car and connected to the 9L gas bottle it should be ok.
You should understand thet you can't run the Coleman directly from the car or any other external power source. You can only use external power for charging the unit.
Mine ran out of power on our last camp after only the first night. It was fully charged before we left home so not sure what happened there. It's certainly done better than that in the past.