How propane works is through evaporation of the liquid in the tank. For this reason, you should never try to turn a tank on it’s side or upside down in an attempt to drain the last drop out of it. If liquid gets into regulators or into lines before it can evaporate, it can do damage to the regulator or appliance. Some tanks, like those on motorhomes and for use on fork lifts, where they are designed to lay on their side, are designed for that purpose, and should never be used in any other position except to lay horizontally.
The evaporation process is affected by many things, the first of which is the surface area of the liquid. Obviously, the surface area of a vertical tank is going to be much less than a large “home style” horizontal tank like is used for household heat. But then, a household furnace can use a lot more propane than small tanks are designed for.
The other reason that tanks are designed that way is because temperature also affects the tank pressure. If you have ever tried to use a small vertical tank in extremely cold weather, you probably noticed that it didn’t perform well. They don’t have enough surface area for evaporation in cold temperatures. This is why home tanks are designed to lay horizontally, and are much larger in size. That extra size isn’t just because the home’s furnace uses more gas (although they do). It’s because in colder temperatures, the tank has to provide more surface area so that enough liquid can evaporate fast enough to meet the needs of the furnace. If you try to use your RV in winter weather, you may find that the furnace can’t keep up, and it isn’t the fault of the furnace. It’s because your typical RV tanks don’t allow enough surface area for the gas to evaporate fast enough to keep up with the requirements of the furnace!
If you absolutely have to use an RV in freezing weather for an extended time, you will probably have to have it outfitted with an extension hose and shut off valve, and then connect to a larger tank. We had to do that one time when parking at a resort in the wintertime in Michigan. We had to get a propane company to bring us a 100 pound tank, which is about four feet tall and about 30-inches in diameter. The larger diameter allowed enough surface area for enough evaporation to meet our needs, whereas the smaller tank on the motorhome, even though it was a horizontal tank, couldn’t keep up with the demand. That’s just how propane works!
The smaller the appliance, the smaller the tank can be. Many appliances, like the ones we recommend for your minvan camper conversion, can get by with the small one-pound tanks, like portable camp stoves use. Larger things, like barbeque grills have a larger burner that requires more gas, and so larger tanks are required. A home furnace has a much larger burner, and is used more in cold weather, so those tanks must be larger yet. Just keep in mind, there is reason they are designed the way they are, and they should never be used in any other position than how propane works correctly!