I installed this type of LP gas monitor on the 3400. It let's you know when the regulator has switched tanks.
http://www.folandsales.com/catalog.a...showprevnext=1
I mounted the monitor low on the wall in the bathroom running the wire up through the furnace register hole. When I run out of LP in one tank and the regulator indicator goes red a light on the monitor flashes letting me know I have an empty tank. The light will continue to flash until you flip the regulator lever to the full tank.
Now I wont have to check inside the LP compartment to see if we have an empty tank or not (I always seem to forget)
I found mine at a local dealer for under $30.00. The install was really easy. You remove the black cap from the top of the bubble indicator on the regulator (glass tube w/ red/green indicator inside) and replace it with the wired cap/sensor of the unit. Mount the monitor inside the 5er and route the wires to the sensor and wire it up.
Phil