2003 Chevrolet Avalanche CHeck Engine Light

Just today the check engine light came on. Since I bought the truck, I had this light came on twice before and it has been caused by the emission control and been fixed by the dealer by changing sensors. Now this time it is happening again. I brought it to the dealer and now the service person is telling me that the diagnose is happening on the gas line sensor which is located inside the gas tank. I was told that they have to bring down the gas tank and replace the sensor. The cost is around $550 plus tax. I am located in Raleigh North Carolina. Tell me if the service dealer is telling me the truth. Note that during the past 2 occurrences, I only paid less than $300 including tax. Thank you