Had the same leaking issue when dewinterizing my unit. Turns out when I added water this year the leak started. The cap had been replaced, put the rubber washer was missing on one. Tried to find replacement washer at both HD and Lowes, which carry Plex tubing, with no success. Went to True vale hardware and they matched it up a washer that goes in the end of a supply line to a bathroom faucet. The faucet fitting has a similar cone shape as the one lost on rv. With a bit of Telfon tape., problem solved. A post in the formin indicated, caps might be ubique. To the RV industry.
Marriot2 on Wheels 2017 Flagstaff Classic 832 OKBS
2013 GMC Denali 6.6L 2500 HD DMAX w\Alison Trans
Pepper Pomeranian rescue 101 days as camping buddy
Days camped in FR, 2011 = 20, 2012 = 57, 2013 = 59
... 2014 = 39, 2015 = 52, 2016 = 68, 3 more planned