I have an RDS 45 gal aux tank. I had it in a 2006 Chev--it didn't like for the tank to be left on while traveling. It would go ape after a while by running the fuel gauge down to empty, giving me the fuel low symbol on the dash along with the chime, and so on. I now have it installed on a Ford--it doesn't care. I leave it on all the time and the gauge just shows that it is full. I have a solenoid valve installed in the line with a switch on the dash. My line is 3/8 inch.
Both the OEM fuel tank and the aux tank are vented to the atmosphere. The aux tank came that way from the factory.