The standard size that yo uuse on grills should work just fine, most are rated at 200k btu per hour
Propane has around 91,000 btu in a gallon. 40 X 91000 divided by 180000 is about 20 hours of cooking at the 180,000 btus per hour. That could be frequent trips.
Actually they are only supposed to fill to 80% capacity which woul dbe 32 lbs and takes you closer to to 16 hours.