Why not tee in to an existing supply pipe fit an isolation valve and run a pipe to a new point, such as the one that is newly fitted to most modern caravans. Nice neat small sliding cover with plug in connection that auto shuts off and on as you plug in and disconnect. I have fitted two on my own m/home and two on a friends we both find them excellent.
I had mine from http://www.jacksonscamping.com most know them as Jacksons of Arley. You will need to know the size of the copper pipe you have fitted and they will supply the tee piece and as much pipe as you need. Best advice is use a competent CORGI fitter who will have the equipment to test for leaks when the job is done. They also do this on the annual habitation check so if you do it yourself make sure it is just before that.
You can buy a kit for doing just that, brought mine back from the US, see campingworld.com, they will ship to you. This way you have the correct size of coupling etc. Comes with 12ft of pipe to connect to the bbq
Never used it, but my extend-a-stay adaptor for the appliance/genny LPG tank (my RV is diesel engine) has a BBQ hose connection which I believe will take from either the LPG tank itself (what you want) or your alternative external gas bottle, depending on how you're set up.
Got it from ABP in the UK (easy to find on the web) - might be worth a try...
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