Tank Filler Pipe And Cap problems of the 2011 Volkswagen Jetta

Bought my Jetta tdi new 4 months ago. Filled up with diesel on a Monday morning and had driven approximately 70 miles before I had this problem. Was driving home Tuesday (at 60mph) when suddenly lost acceleration and glow plug light started flashing. Was able to limp to a vw dealership about 1 mile back but car would not accelerate above 35mph. The next day the dealership gave the diagnosis of water in the diesel and told me "everything that the diesel had touched" would have to be replaced (fuel tank, lines, filter, etc. ) . They said the vw warranty would pay for it this one time but if it happened again I would be responsible for the over $5000 cost. When I asked how I could avoid this in the future I was told I just needed to purchase my diesel from a "reputable" place. (seriously!!! do they think I am buying "bubba's home brewed biodiesel" on the side of the road or something???) I reported to the texas dept of agriculture that the vw dealership was claiming the diesel I had purchased on Monday had water in it. The tda promptly came out and investigated the station where I purchase fuel and reported to me that they found nothing wrong with the diesel. This sounds to me like a major oversight on the part of vw if there is no way to avoid such a potentially dangerous and costly problem. I traded a much nicer and more comfortable SUV for this Jetta in an attempt to save $$$ on fuel. I think I've made a huge mistake.