One of the pex 90s that feed the cold water inlet on my kitchen sink decided to leak. Hand tightened it and more leak. Turns out the cone washer that seals the 90 degree connector to the faucet cold water inlet had compressed too many times and was deformed. Lucky we had a great mobile RV tech right behind us in Johnson Creek RV & Resort who came by and fixed this issue. Don't think you will find these seals/washers in a box store but not sure. Tech is a vet and his name is Steve;
www.hcrvs.com - he stays in one place for awhile then moves on so check to see if he is in your area if you need immediate help. I had him change the washer/seal on the hot water feed as well. I looked at various RV parts places online and didn't see any of these seals. It has been a weird camping outing have had the heat on and A/C within a matter of a couple days. We head back home tomorrow. These seals are something I want to locate and keep a couple as they are the same as the ones used on the bathroom sink and perhaps the tub. These parts may also be called "cone seals".