Your apparent open circuit would lead me to first check all wiring and connection integrity between the guage and the tank. That possibility absolved, the tank unit is about the only thing left.
As a subnote, last year, after a lifetime of often difficult wiring diagnostic adventures, I broke down and bought a Sperry Lan Tracker, model ET4220L, at either Lowe's or Home Depot, I can't recall. It is a dandy device that allows me to check continuity in long circuitous wire runs with relative ease; it is a two-piece setup where one part sends a signal from a distant wire end, and the other part measures the existence and degree of signal at the other end. It works with AC, DC, coax cable, or whatever (I used it to identify which TV coax, in a junction bundle of 7, came from which room in our house). It would be nice to have in your situation, and it probably wouldn't be the only time you'd ever use it. There are other brands and variations out there, but I liked the Sperry's compact convenience and reputation.
-Joel
