Help with minor plumbing leak at fresh water connection

I've got a leak in my trailer where the connection is made between the trailer's plumbing system and the city water connection. The inside compression fitting is leaking. I've obviously tried retightening it but it still leaks. The connection is marked as "hand tighten only" so I've resisted the urged to take a some channel locks to it and really torque the heck out of it. Being all plastic I'm sure I'd damage it anyway.

I know nothing about plumbing. The plastic fitting that is leaking has a small gasket inside it that's kind dome-shaped. Is this a replaceable part? Or do I need to cut off the fitting and replace it somehow with a new one. Links to whatever parts I may need are appreciated.