Chris,
I'm not familiar with the CC model's, so can't help you with the access question, but SeaCraft was always real good about putting hatches over the fuel tanks so you don't have to rip up the deck to get to them. Sounds like you have two tanks? Accessing the fuel tank is a piece of cake on a Seafari or Sceptre. I would think a previous owner would have installed an access plate over the sending unit so it could be changed without pulling the console or hatch.
Whoever installed your senders obviously got the polarity reversed because your gauge is just reading voltage across a variable resistance. If the voltage is backwards, the gauge will read backwards. I just replaced a Tempo sensor that died after only about 7 years with a Wema sensor.
http://www.wemausa.com/sensors/cover...l#.UtiJXvRDua8 They're a bit more expensive but if yours is hard to get to, I would definitely recommend them. They're aerospace quality and use magnetically actuated reed switches that are orders of magnitude more reliable than the old mechanical arm wiping across a resistance coil!