Paul.
As a "converted " LPG user both for towing and regular use I would say that as a towcar the Fronterra would be no different in power or drivabilty than the petrol version.
I have had a quick look on the net at LPG equipment suppliers and there is 60 litre under floor tank quoted for the Fronterra LWB so it is possible. However if you don't need all the boot space I would recomend a 100 litre inside tank. 60 Litres is only going to give you a range of 200 miles max.
Use an LPGA approved installer if you are contemplating conversion and go for a state of the art sequential injection system, expect to pay about 25% more for an underfloor set up; you should get your money back on the cost of conversion in about 18 months if you do 15 - 20k miles a year and of course cash flow is drastically improved - fill the tank for