Nicholas,
If memory serves me right the sending unit is on the drivers side of the tank a few inches down from the top. There should be some nuts on studs that stick out of sending unit. The wires go the the back of the fuel gauge. If making sure everything is clean and tight on both ends doesn't fix the issue it could be the sending unit itself or the gauge.
Sending units are known to wear out. I had to replace mine and it shows the tank to be completely full for the first two hundred or so miles. By the time it drops to 1/2 tank it is fairly accurate. I really don't let it get to far below 1/2 so I don't know the accuracy there.
The gauge is simply reading a voltage level from the sending unit. The voltage changes as the float arm swings up and down. You can take a screwdriver and touch both post on the back of your fuel gauge to see if it swings all the way to the full position.
Hopefully this will make sense and help you some.
Thanks Carl, I haven't had a chance to crawl up in there. I had the Beaver in an RV storage lot with just 30A service and its been fine the last 2 years but the place began having issues with electrical service being inconsistent, as a result I kept seeing irregularities with my power system and the Xantrex 3012 SW.
Finally it came to a head and the unit wouldn't charge the house batteries. Everything else seemed to be working fine, just no charging. I called tech support over at Xantrex, a great service btw. Those guys are a fountain of information and I think they get paid by the minute. I was on the phone with Jeff as we went though numerous tests, setups and resets. Finally we got to the point where the suggestion was to completely power down the rig for and hour. I unplugged it and disconnected the house batteries and let everything sit for an hour, hooked it all back up and plugged it in and viola, power came back up and the nearly dead batteries were charging quite vigorously.
I'm planning on getting it in the driveway and crawling in up next to the tank in the next couple days and will report back my findings.