Greg, the legal tanks now have an Overfill Protection Device (OPD) fitted to the central hole. This is a float valve. The original level sender unit also used a float valve. So that the two float valves do not interfere with each other, the legal tanks with OPD and level gauge facility have TWO threaded holes, a few inches apart. The tanks are sold by Worthington, to name one company. Your tanks are probably like mine, with a single hole. I can't bring myself to spend a lot of dollars on new tanks, so I rely on the red/green flag on the changeover regulator to tell me when a tank is empty. Nick.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."