start at one end of the line and work you way towards the other end. A q-tip makes a great moisture tester. Grab a big handful of them so you have a fresh one to test each join you come to. Press it against the joint then get in the light
and press and roll the end of the q-tip to a piece of white paper to see if there is a moist blot on the paper. Of course there could be a leak in the line itself so save that test for last if you don't find any leaks
at the joins.
Any fitting of any kind in a water system can leak. The seals on the pump can also leak. But a lot of experience with RV water systems is not needed to track it down to a specific source. It just takes working methodically and patiently.
A lighted inspection mirror is a great help for working in confined spaces where it is difficult to get a direct view of things. You can find those at any hardware store.
Remember water moves and runs with gravity so the leak could be further of the line that where you find moisture. So be sure you don't just stop at one spot thinking you have discovered the one true source, continue on and do checks at other joints on up the line to see if they are dry. And of course it is always possible to have more than one leak which is one of the reasons plumbing can be frustrating.