Qest fitting plumbing issue

We recently purchased a 95 Nash 26J. I've discovered a very minor leak, where the hot water comes out of the water heater.

For now, ignore the red lines I've added to the photo, I'll get to those in a moment...

These SEEM to be the old style Qest fittings. The size of the pipe is 1/2". The 90 degree elbow, that goes into the heater, is my issue. It's maleXmale threads. It threads into the water heater and on the other side, the female adapter threads onto it. The Female Adapter/90 degree male threads HAD an rubber 0-Ring in it. I say HAD because when I took the joint apart, the 0-ring came out in pieces.

Here's my issue...I thought Qest fittings used rubber beveled washers, NOT O-rings.

Since there's an O-ring, I have a few questions...

1. Any idea what size this O-ring would be? I am sure that my local RV Service Center would know but they aren't open to day and are a 30 minute drive from my home. Lowes is a 10 minute drive...lol.

2. Where exactly was this O-ring placed?? Did it go inside the female side? It kind of looks like it may have went inside the male side??

Now, let's get to the red lines I added to the photo...

If I'm unable to fix this, with an O-ring replacement, I've considered cutting the Qest lines, at the location I've added the red lines. My issue is, do I replace it with PEX, PVC/CPVC or what?