1. You need a better workshop
2. If it worked properly before they dropped the tank I would say they stuffed up the wiring during the install and now will not accept their mistake, make a complaint to "fair trade"
https://www.fairtrading.nsw.gov.au/
3. What did they do to diagnose the issue that they "couldn't work out"?
4. You need a better workshop
Or;
E = Enough
F = Farkinneedpetrolsoon
Or;
Set aside a day for you and another to drop the tank, dropping these, without a hoist, using jack stands, is rubbish, but it can be done on a hoist in 2 hrs, well, that's how long it took my joint to replace my fuel pump
Does you low fuel light come on when 100% full, or near empty?
I'm no auto elec, but in my uneducated over opinionated opinion, it sounds like the wires have been swapped around, maybe