If a conversion like this was done by someone knowledgeable and by someone that has experience with US vehicles, then they run fine. Usually you keep the OE gasoline tank and a CNG/LPG tank is added somewhere, in your case they replaced the spare tire with it. Sometimes these conversions are a crapshoot when the conversion shop tells the owner things like "don't tow on CNG/LPG, have it run on gasoline for that" because you then know they just slapped some standard tune on it and called it a day. Adjusting the injection times and other parameters of the gas control module takes some time.
These cars also require something called FlashLube so the valve seats don't burn away, unless the cylinder heads were redone and equipped with different seats. A stock 5.3 is not gas-capable from what I know. The spark plugs are usually changed to a different heat number.
Here in Germany CNG/LPG gas stations are rare, I only know of one (1) in my area. That is probably the reason why "gas cars" never really caught on, even though the German brands offered them equipped like this from the factory. They said that since gas burns much cleaner than gasoline you can extend the oil change interval and so on.
Mine runs on good old dinosaur juice, I don't drive that much so it's ok.
