Bailey 745se Approach.

I had an Approach 740 (now got an Alliance) Can't answer your question but if you remove the fridge vents I imagine you should be able to see the feed pipe. They are probably 8/10mm. As I doubt they use different size pipes you can also see the pipes under the cooker where the isolation valves are. There is an excellent Facebook Group called the Bailey Motorhome Owners group which might be worth joining.

David

Or play safe and let a gas fitter make the decisions?