You mentioned that you're hooked up to city water. Do you have a pressure regulator in the water feed line? if you don't, you could have higher pressure than the RV's plumbing system is designed to handle and the additional pressure caused when the hot water tank has heated the water may be just enough to start a leak at the weakest point.
Another possibility at the RV toilet is that the leak is not where the water line attaches to the toilet valve but is the valve stem. The valve stem is sealed with an O ring which can fail. When this happened on my toilet, it leaked for over a year into a basement storage compartment before I realized what was leaking. (I thought the water in the compartment was from the roof or the city water connection.) When I discovered that it was from behind the toilet, it still took me over a week to locate the very very slow drip from the valve stem because it's in a location that's almost impossible to see.