This is the most compact low pressure regulator that meets ABYC marine standards, with gauge for required LPG system leak test.

Fully assembled and leak tested
Solid brass: male POL tank fitting, street tee & 300 psi leak test gauge (low profile, water tight, stainless steel)
UL Listed die cast zinc body with 3/8" female NPT outlet


Heavy on the technical side...
Thinking back to your high school chemistry class because of the physics of your propane tank simultaneously containing both liquid and gaseous propane, the pressure inside the tank remains virtually constant (varying somewhat with the ambient temperature) until all the liquid propane is gone and the tank is functionally empty. For this reason, and unlike gas-only SCUBA or CNG tanks, the pressure on the gauge cannot be used to determine how full the tank is. The pressure gauge is there to perform periodic leak tests to check the integrity of your system.
We recommend that leak tests be performed every time the tank valve is opened, and at least every two weeks when the valve is left open during extended use.

To determine how much propane is left in a tank, you have to weigh it and then subtract the known weight of the empty tank.
Or, with a composite tank, you can just look at it to see the level of the liquid propane in the tank. This feature, alone, probably goes a long way toward explaining the huge popularity of composite propane tanks.