Welcome aboard!
A 5 lbs tank has about 91KBTU of available heat. Most RV furnaces are rated at 30KBTU/hr and are only about 70% efficient. So you will burn up that 5 lbs tank fast. Just remember, typically your furnace will cycle on/off, so it will not use the full 30KBTU each hour.
In a PU, you will go through a lot of propane for heat, as you only have a piece of cloth between you and the elements. On a 30 degree night, you may use most of a 20 lbs tank, especially if the daytime high stays on the cooler side, and what setpoint you like the thermostat at.
Our old PU did not have a furnace. When it is that cool, we tried to have an electric site and use a 1500 watt space heater. It helps alot.
Currently we have a HTT. We still use the same electric heater, and have added electric mattress pad heaters. Helps a lot, we often just turn the furnace off at night. In the morning, flip the furnace on to bring the HTT back up to temp.
We have a lot of camper friends most say they can get 1 -2 nights out of a 20 lbs tank in their PU. I think a lot depends on the thermostat setpoint and if they supplement with electric heat.
I would say a 5 lbs tank is to small for a cool night in a PU.