Why do you want Propane? Propane when burned has water vapor in it. (propane and O2 = Co2 and H2OV. Depending on your burner you will get CO, NOx, and Carbon. That water vapor is acidic and will condense on your food. Flue product condinsate is about as acidic as apple juice and working with boilers, Condinsate will eat steel, eat concrete, and having gathered some of it in condinsate traps it smells aweful. I can not see how it will help your rub.
A million BTU boiler will produce 10 gallons of condinsate per hour of run time. Your smoker is going to create about 3 gallons of condinsate for 12 hours of smoking.
If you are looking for ease I would get an electric, only water and wood chips and an even heat source.
Other than that it is hard to be the $90 webber silver kettle grill and a couple of bricks as a smoker. You can also go the Alton Brown way and use $40 woth of clay pots and an electric hot plate. Good eats on Food network. If you want I will find out the eposide and a parts list.