I used a turkey fryer propane burner with my 5 gallon stainless pot, and while evaporation was slow, I was able to (mostly) get a good rolling boil.

One thing to watch out for is the outside temperature - as the ambient temperature drops, the vapor pressure of the propane drops as well, and it's harder to get a good, hot flame going. With this evening's temperature hovering in the mid-30's or below, that will have a noticeable effect, and especially if your tank is running low.

Also, as mentioned above, keeping the wind down is a good idea.