RE: Propane Tank
Jeff is absolutely right.
There is confusion because we interchange the words cylinders and tanks" but by code, a cylinder is "A portable container constructed in accordance with U.S. Department of Transportation, Specifications for LP-Gas Containers (49 CFR)" They are typically used for BBQ, vertically mounted on travel trailer frames, outside of homes where natural gas is not available, etc. I believe cylinders are subject to DOT static testing 12 years after manufacture date, then 5 years thereafter but specific requirements vary by state.
Tanks are containers constructed in accordance with the Section VIII, “Rules for the Construction of Unfired Pressure Vessels†of the Boiler and Pressure Vessel Code" published by ASME. Tanks are not subject to static testing, only to good sense safety practices. If it looks old, dented, damaged or rusty, replace it.
What is the difference? As far as I can tell, cylinders are constructed from a lighter gauge material than tanks, therefore, they are cheaper but will deteriorate sooner than tanks, hence the requirement for static testing. Most cylinders are designed for vertical use however, horizontal use cylinders can also be purchased but I think theyrerequired to be transported vertically so I guess they're no good for RV's.
If your RV has a cylinder, NFPA 1192 limits you to not more than 3 cylinders with individual capacities of 45lbs of LP. If your RV has a tank, NFPA 1192 limits total tank capacity to a maximum aggregate water capacity of 200 gal. (Don't ask me to explain that why they use different terms).
As for NFPA 1192, not only does it cover the propane rules but many other aspects of RV manufacture and safety issues such as plumbing, holding tanks, heating, AC, exits, vents, smoke, CO & LP detectors, etc. (RV Electrical is covered in Article 551 of NFPA 70, the National Electric Code). I have the 2005 edition of NFPA 1192 but the latest edition is 2008 and a new edition will be out in 2010. As far as I know, it is unlawful to copy and post this or any other NFPA code so please don't ask. Although NFPA is a "non-profit organization", they do not give away their materials and information. Fire departments and code enforcement agencies have to purchase their materials. NFPA 1192 on DVD can be purchased fo about $35. I highly recommend having a copy of NFPA 1192 and NFPA 70 available for those rainy days when you are totally bored and wish to learn more about your RV.
Sorry for the long post.