Need help/advice finding INTERMITTENT water line leak

I suggest you find/fix the leak rather than mess around with trying to relieve hot water expansion. Further - if you have a loose pipe joint who's to say it's not going to just give way one day and if your not in the rig your neighbors are going to watch waterfall from your rig. That's one of the reasons some prefer to use water pump and some will never leave the rig until they turn off the shore water source.