As stated above push release ring toward fitting body, so there is no gap between release ring and red arrow.
Try using an open end wrench over the tubing to push against the release ring. Then push the hose in, before trying to pulling it out.
Cut 1/4” off end of tubing, wipe off end of tubing, making sure all traces of dirt and any paint are removed from tubing.
Reinsert tubing into fitting, while release ring is back toward its normal position. Push it in until it stops. Marking the tubing a 1/2” from end with a Sharpie will help you see how far you have pushed in the tubing. Once it is in all the way, pull outward on tubing to engage the O ring around the tubing
Dirt will get between ring and tubing, chafing the tubing. And with constant pressure on the tubing at the O ring, the tubing will have a slight indentation where mating to that O ring. Most of the time, a clean smooth section of tubing is all that is needed to eliminate the air leak
Finally air up system and spray with soapy water to verify fix.
I hope it as simple as this to fix that air leak.
2008 Monaco Camelot 40' PDQ