First off welcome to the Newmar forum Paintblend.
As you may have guessed by now Newmar doesn't supply much info on plumbing or wiring for that matter for their coach's.
This link will provide some idea of how a system might be set up. In general the four Newmar's I have had have been setup the same for plumbing.
What I would do is see if I could locate leaks with the water turned on at low pressure to spot drips if hidden would have to trace piping from water closet with cover removed if possible. Your fresh water valve to tank would be a start than to water filter if you have one and feed to tank. Most plumbing in that area is exposed feeds to all other plumbing faucets and toilets would be more differcult to find the leak.
Your OP says you think may be feed to water tank so the valve would be the place to start.
With the diagram in link you maybe able to add your plumbing runs to it for your own reference.
Let us know how your making out and if we can continue to help.
Congrats on your new RV.