Easiest way to trouble-shoot a gas gauge?

Bill,

It very well may be the voltage limiter, they fail all the time and it would certainly be common to both tanks. There are numerous blogs about replacing them with newer solid state units or building your own using a 7805 regulator. You might be able to pull the instrument cluster out and check the regulated voltage if you can get at it. Since I don't have a schematic I can't help much more. Let me know what you find.