Mine takes 65 litres to fill from empty (you know when it was completely empty when you hear the pipe between the filler and tank fill as soon as you put the filling nozzle on) and I usually expect to get anything between 180 running around town and 220 miles on a run. Towing can make a difference depending on what I've got hooked up behind. It doesn't seem to be the weight that affects it, more the wind resistance. Towing a low sports car on a car transporter trailer has very little affect on range, towing a tall box trailer really hammers it even though the car transporter could weigh up to 2.5 tonnes and the box trailer could be empty.
My personal best, which I have never been able to beat since was driving from Germany, through Holland and into Belgium. Wasn't towing but the traffic was heavy so travelling at around 50-55 mph. Expected to run out at any moment when I got up to 220 miles and was still 18 miles from the next filling station but pulled onto the forecourt with 238 miles showing on the trip. Was absolutely astounded that the pump shut off at this:
238 miles and still had 10 litres left in the tank!! 19.5 mpg but at Belgian LPG prices that works out at well over 70 mpg. Thought the pump must have cut off early and I hadn't got a full fill but the next tankful lasted for as long as I would have expected. I've had other occasions when I've stopped to fill as I must be getting low and it has taken less than I expected but this was the one that I actually took notes of. I took the photo more to show by how much we get ripped off on fuel prices here.