The guage probably works on an earth system mate...

Though I couldnt tell you how to supply a feed to the actual guage to check it.


BUT..... Exactly the same happened to an RST I had, was sat around for a while, as yours with half a tank...

no matter how much fuel put in, gauge didnt move.......

SO, dropped the tank down, unscrewed the sender, lifted it out and it had corroded quite badly, seized in fact, the inside of the tank had rusted up like a goodun too, it was obviously the condensation

picked up a S/Hand tank, and gauge fitted and all worked fine..... If you do a search, I posted up a little while back a step by step guide to removiong and refitting tank.

The only confusing bit is the fact you mention a new tank a short while ago, but other than that I cant help any more mate sorry