10 ohms, sender to any ground, is about right for about full and shouldn't read past the full mark on the gauge. I'd still be interested in a resistance reading from the gauge lead to a good ground as it sounds grounded there. If it's the same 10 or 11 ohms based on which tank is selected, run them down and see what happens. If it's lower at the gauge than at the sender, ya got something grounded that shouldn't be. Tank selector switch issue?