Ok, well I have no idea what the dude did. 9.5mpg sucks. But in the perfect installation, you have to run on fuel(gasoline) for a minute during cold starts to warm up the engine block and prevent seepage. After the thermostat has opened up then you should switch over to the LP system.
I think your installation is flawed, not even knowing what he did. You wont know but you can ask the dude who looks at it, if there is a fuel pump remote shutoff? This could be a toggle switch before the relay to the pump. This will prevent pump burnout. But in your case, I think the greatest problem you have is the system isnt tight. Your mileage sucks because there is an air/fuel error or vacuum leak in your system. He is supposed to drill a hole into the air intake plenum before the throttle body to secure the metering jets or a better way is to drill a hole into the intake manifold for better fuel distribution. I think your guy cheaped out in the install and just punched a hole on the air tube and stuck a jet in there. And the nozzle on the jet may be too big. Take a look, see if he stuck it in there with electrical tape or some BS. Check the routing of the LP fuel line, hopefully he didnt run it over the exhaust manifold(this is a 4cylinder right?).