If you can see rust in the fuel filter, or on the tap screen after removing the tap, it could come from contaminated fuel but its much more probably from the tank. If you can see rust when you look through the filler cap or any of the other holes (using flashlight, inspection mirror), then obviously its rusty.
The only other avenue of investigation I can think of is fibreoptics, which Ah Huang may not have to hand.
If you can’t confirm the rust, then “another shop” cant either, so taking it elsewhere seems a waste of time.
If it hasn’t got a hole in it, you don’t need to fix it. It may, however, be worth cleaning it.
I’d try an abrasive scouring of the inside, to disloge troublesome rust flakes before they dislodge themselves. A length of chain swooshed/clunked around is probably easiest to remove afterward, though shipyard confetti or even gravel are also sometimes recommended.
I used detergent and hot water on myy Zing tank, hosed it out and then rinsed it with ethanol followed by acetone to de water it, but that may be more trouble than you want. The acetone can get you quite intoxicated unless you’re in a well ventilated area (I was in the shower) and may not be very healthy, but it seemed hilarious at the time.
People use acids (hydochloric or phosphoric) to aid rust removal, though I didn’t, since I was mostly concerned with “gunge” rather than rust. HCL is available here as a bathroom de-scaler but I’d want to dilute that a lot. Dilute Phosphoric acid is available as CocaCola.
After cleaning, or if you buy a new tank, put a bit of ethanol in it every month or so to de-water it, and change the fuel filter.