Can't fill my built-in propane tank

Recently I have tried to get my built-in propane tank filled in preparation for camping. At two different propane filling stations, the propane would leak from the propane connector that screwed onto my tank. My tank fitting is clean and looks new as I keep the cap on it when not in use. At first I thought that the O-Ring on the propane station's connector at the end of the hose was faulty. However, the same thing happened when I brought the MH to another place to fill it up. Both filling stations just sell propane as a sideline and are not fully in the propane business.
Could there be a missing gasket on my tank fitting that causes the leak? The leak only happens when trying to fill the tank. I will bring the MH to a propane only business where they have propane mechanics if I can't solve this mystery. I have filled the propane several times before with no problems. Thanks for any help you can provide.