Is your 2014 Challenger a model 35H? You mentioned previously that the kitchen is not on a slide though and 35 H does have that…..but cupboards are on the left (passenger side) rear?
Are you getting the leak on city water feed or when you are on coach water pump….or both? You say the leak/hissing sounds like it is in the rear bedroom cupboard area. Sometimes access to hoses etc can be obtained by removing lower cupboard drawers. Pull out and lift them up at the front off the roller guides. If they come out use a flashlight and look down in the area where the hissing occurs. If no drawers in that area look for a false floor in the cupboard floor. It might have screws in the corners to hold it down.
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