Iím far from being a plumber or a gas contractor so take what I suggest as such. Are you able to take a leak check on the entire house, to see if you have any leaks? Or do you know that you indeed have a leak in one or more of the areas that you canít get to? I know this is basic, but can you get soapy water to the fittings you canít reach?
I have use a helium detector to look for water leaks in underground pipes before, but I think you would somehow have to get a above the fittings for that to work.
Hope I could help!