If you THINK you have a gas leak, then there's a simple way to find out. Charge the system with gas, remove the gas lines from the kegs (pull the disconnects off the kegs, leave the disconnects on the lines) and then turn off the tank. Mark where the high pressure gauge reads and then walk away. Check on it in 30-60 minutes. If the needle hasn't moved, let it go overnight. If it still hasn't moved, let it go to the same time tomorrow. If it still hasn't moved, you're leak free. If it has moved, then you need to hunt down the leak and fix it.
I just went through this exercise myself. It took charging my system to over 20psi before I discovered I had a leak. I've spent the past few hours checking, tightening, and changing connections. I'm 99.5% sure the system is leak free now. I'm simply letting it go a bit longer before I certify it as leak free. If you charge your system (off the kegs) to 20-30 psi, any leak should become very evident. Even more so if you turn off the tank once charged.
I also experienced the high pressure gauge display difference at different ambient temperatures. When I first put my CO2 tank in the fridge, I thought I had a major leak since it dropped a few hundred PSI. Come to find out, I didn't... I've since placed the tank outside the fridge, running through a bulkhead fitting. It's a bit more work to setup, initially, but at least I can easily see where the tank pressure is.
BTW, BobbyM has a thread about carbonating kegs on gas... I would suggest reading it. Very useful information as to the different techniques to carbonating in kegs (with gas)...
