I have heard that as long as the have liquid CO2 in them the pressure stays about the same? Is this true?
Correct. Until you get to the running on fumes stage, tank pressure is a function of temperature. On the other hand, when the gauge hits 300 psi, start planning on getting more. With two tanks, I just run one down, then stick it in the van. My supplier is 25 miles away, so I'm not willing to make a special trip, but I get out there 3-4 times a month.
Even with a #5 tank, the fumes will push a couple cornies or carbonate one.