If you don't have a leak in the line, you have a leak in the tank or the keg. I would suspect the keg first. Pull the gas lines from all your kegs, then crank up your pressure, and finally turn of the CO2 on the tank.
Wait a day and see if your regulator is still reading the same pressure. If it is, your leak is definitely in your kegs.
You can look for leaks in kegs by using some starsan or soapy water in a spray bottle and looking for bubbles where you spray it.
I used this technique to find out that one of my corny kegs wasn't completely sealed. I opened it, added a little keg lube, then hit it was a burst of 30psi pressure to force it shut.