I had this problem with my 2002 Montana. The fuel gauge inside the tank was defective I went to a garage where and had the gauge changed (you can buy the part from the dealer ) You have to take the fuel tank down and unplug the wires and the fuel lines(you need a special tool to disconnect the fuel lines) Parts and labor cost me $260.00
Either the gauge is broken or the sender is broken.
Bad connection, blown fuse, or the float in the tank is stuck.
sounds like the fuel sensor inthe fuel tank which means removing tank to replace
you will need to replace the fuel sensor and it is all in one with the fuel pump its in the gas tank.
The engine coolent may be low enough that sensor is not surounded with coolent when engine is not running.
Get the head gasket problem fixed.
The thermostat is probably stuck in the open position.
Either your engine coolant temperature is too high , your oil pressure too low , or your gas gauge is near or at empty
Either your temperature gauge is reading too hot , your oil pressure is too low , or your gas gauge is at or near empty
Either the sensor is broken or Your gaug on the dash is not working.
you either have a diying battery or a ground short
Either bad gauge (unlikely) or bad sending unit (probably). Either way you're going to have to pull the gas tank to diagnose the problem. Either that or top it off every 200 miles and forget about it.