Like has been said, the grill can only have one regulator in line. Since your grill has a regulator attached to allow it to be used with 1 pound bottles or bulk bottles, you MUST attach it to your trailer's tanks BEFORE the trailer's regulator.
OR
Do what I did and remove the regulator from the grill. Then you MUST attach it AFTER the trailer's regulator, I.E. to the trailer's propane outlet connection.
In order to retain the ability to use the grill with a stand alone bulk propane tank, I bought a propane hose with an inline regulator. I use either the propane hose WITH the regulator or the propane hose WITHOUT the regulator depending on where I want to draw propane from.