It depends on some factors:
Will you be running with your water tanks full?
Will you be loaded to the max with luggage and people?
If you are, 80 lbs.
If you're running fairly light, I wouldn't go much lower than 75.
Underinflated tires experience a lot of flex in the sidewalls during the course of driving. This does, as mentioned, cost extra fuel, plus, the flexing builds up heat in the tire. In warm weather, with enough weight on, that heat builds and builds until KAAABOOOM!!!!. If you like buying tires, go for a nice ride. If you want your tires to last, sacrifice the ride and your tires will thank you for it.
I've seen too many truck tires turn into rubber alligators due to running low on pressure.