Recreational Vehicle Lpg Tank Valve/gauge problems of the 2008 Mercedes Benz GL450

I noticed a strong gas smell after a full tank of gas with my Mercedes Benz Gl450 after I received it back from the Mercedes Benz dealer to replace the driver side fuel sending unit to fix the broken fuel gauge. The strong gas odor was non-existent before the repair. Initially, I thought it was just a residual smell due to the repair but after a week when I filled again, the gas smell was still there. The smell is so bad especially in the morning when I get inside the garage. The smell is evident on the rear passenger side close to the gas tank. On January 11, 2016, I brought back the vehicle to the same Mercedes Benz dealer with a quarter of a tank to investigate the problem. After a couple of days, the service advisor told me that the technician/mechanic did the inspection and diagnosis but did not find anything wrong. I was asked if I overfill when I put gas and I said no. If Mercedes Benz could not find out what is wrong to an obvious problem, I don't know what's going on with the vehicle. As I have said before, this problem was non-existent before they dropped the tank to fix the fuel sending unit. I find it wrong that I have to shoulder the burden of worries and uncertainty to a safety problem that Mercedes Benz could not accurately diagnose. At this time, I do not really have a peace of mind safety wise whenever I drive this vehicle with my family.