Valve placement and the use of dip tubes vary with the intended use of the particular tank. Liquid or vapor withdrawl as needed.
The constant factor is that the relief valve is designed to be in communication with the vapor only. In the case of small (4-30# is usual for us RV'ers) tanks with vapor use valves, the relief valve is built into the gas use valve on the side opposite the connection for use.
Portable motorfuel tanks, like those used on forklifts are designed to be used either upright or horizontally. The relief valve is a seperate fitting and will be in the vapor space if vertical orientation is used as well as being in the vapor space when the tank is properly oriented on the forklift in the horizontal position.