Burkholb.....Here is a link to the Centroid site
http://www.centroidproducts.com/moncal.htm. Usually the sending unit is covered in undercoating spray and needs to be cleaned off. It's been a long time since I adjusted mine, but there are some screws on the top of the unit. One is for empty and one is for full. I don't recall if they're marked. If they aren't, call Centroid, they'll tell you which one is which. DON'T mess with the empty one, it would be too hard to correct.
Fill your coach and take to a level spot. Make sure the wires to the unit are clean and tight. Give the "full" screw a turn and then check your gauge with the ignition on. You have to give it 30 seconds to settle. I knew that when mine was adjusted to about an 1/8" past the full mark, it was a good spot for me. That spot meant that at a half tank I had used about 40-45 gallons which would be realistically a half tank of useable fuel.
I adjusted mine when new and then it acted up like yours in year 6. Centroid said I had algae issues....I knew I didn't. They wanted me to replace the unit. I decided to adjust it again and that fixed the problem. It's worked fine since.