Could I hook up a propane gas cylinderfor a grill or RV to my natural gas water heater?

Most water heater manufactures do not recommend converting them. An improper field conversion from one type of gas to another could cause potentially dangerous conditions that may cause an explosion or fire resulting in property damage, bodily injury or both. A.O. SMITH WILL ONLY ALLOW THE CONVERSION OF GAS WATER HEATERS FROM LP GAS (PROPANE) TO NATURAL GAS. THE CONVERSION OF A GAS WATER HEATER FROM NATURAL GAS TO LP GAS (PROPANE) IS NOT ALLOWED UNDER ANY CIRCUMSTANCES. <a href="http://www.hotwater.com/bulletin/137R2.pdf" rel="nofollow">http://www.hotwater.com/bulletin/137R2.p…</a> Hope it helps.  more