While I appreciate the desire to accomplish this task electronically, I think your cheapest and best bet is to use a simple pressure gauge connected to your houshold plumbing. I am assuming that your system is not pressurized, but is a simple gravity system. A pressurized system could still use this principle, but sensitivity of the gauge display to water level would be reduced.
You don't mention the elevation or height of the tank, but if we assume the tank is 15 ft. above your kitchen fixtures and the top of the tank is 20 ft. above the fixtures, a gauge plumbed in at the fixture level would read 6.9 PSI with the tank empty and 9.2 PSI with the tank full (with no water being drawn). In this case a 10 PSI gauge with a couple of hash marks painted on the gauge face would provide adequate resolution to show the level in tank. Cost: from about US$10 up, depending on how fancy a gauge you want plus a few plumbing fixtures.
Note that this will provide a reading on the actual level of water in the tank, not just at a few instrumented positions. If you need an alarm or an automatic pump cutoff at full, a float switch is an ideal device. Just yesterday I purchased a new polypropylene float switch of the type mentioned by JimB from Grainge.com, a major industrial equipment supplier in the U.S., for US$12 to turn on a pump emptying a bucket catching rain leakage through a roof in a workshop. Most of these float switches can be changed from open on float rise to close on float rise by simply reversing the orientation of the float on the shaft. Depending upon pump current draw, you would probably have to add an electromechanical or solid-state relay or a semiconductor switch because the float switch typically can only drive a 30 VA (volt-amp) load.
awright