The tanks should be plastic so there shouldnt be any rust in there.12000 is way over the top.5 or 6 hrs to drop the tanks and flush system should be plenty.as a check out charge they cant charge you more than what you agreed to when you dropped it off.i would get a second opinion.open up the fuel filter and see whats in there.dont forget about the genny filter.if it is fuel contamination its in the genny too.
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