With those types of vehicles, because of the angles they climb and G forces involved with bouncing around, gas carburettors often have issues with metering the fuel due to the floats in the carburettor being effected. Gas flow may also be effected when air is drawn in due to the fuel rolling to one side of the gas tank starving the pickup. Gas is also is more likely to leak in a rollover. Fuel injection solves some of these problems, but propane being a gas has non of these issues.