This may not be the best way but, what I would do is get ready to watch under the sink as the tank is filled and see with a flash light etc. were the water is coming in at. First check and see if any fittings are on that line and check them for a tight fit. Water maybe be coming in as you think and follow a line to a low spot and than drip there. Where never you find water look up, since it does only run down to find the leak.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed 3.55 rear end lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.