The plastic fitting is a Flair-It. It has broken (see the brown mark at the split) and needs to be replaced.
Flair-It catalog is here
. You want to look at pages 12 through 16.
Regular PEX crimp fittings would work, but I would not want to use a crimp tool in that restricted space. A Flair-it fitting can be pried off and replaced without tools.
There appears to be a check valve between the Flair-It and the tank. If this is the cold water inlet, you can:
1) eliminate the check valve, or
2) remove its innards, or
3) replace the check valve, or
4) do nothing with the check valve, and have it fail open or closed at a later time
With those mineral deposits I'd be surprised if the check valve is not stuck partially open.
If it's the hot water outlet, then replace the check valve. It's part of the winterization bypass system. At a later time you could modify the bypass system to eliminate a heater outlet check valve; see this thread
Plumbing diagrams for your coach, among other useful info, are available at this link