Anyone running Propane, or thinking about it?

I have a 2011 Ford F250 CNG van. Has 2 tanks, I dont know the capacity but I believe its 20 "gallons" on the pump, and the pump displays "gasoline gallon equivalent" or about 5.66 lbs of CNG. I watch the odometer religiously as getting Dr Hook to take you in is a major gaff. I start planning to head for 1 of 2 stations here in Long Beach at about 118 city miles and I am panicking at 138 miles (once). When I looked under the van at the pressure gauge at 138 miles, I was down to 500 psi from a 3600 psi tank at %100 fill. Trouble is the guage is very sketchy. Itll show 1/3 of a tank and 5 minutes later itll be pegged on E. I dont know if they ported the pressure to fuel gauge over correctly but I have learned not to trust the gas guage. Its 2.22 a GGE here so I dont understand how the poor mileage equates to CNG if we are talking 2.22 for an equal amount of energy as a gallon of gasoline? Ive never run it "dry" so I dont know actual mpg Im getting. Id do it in a heartbeat as I know where all the stations are but not for my road trip car.