If the gauge and sender are working propperly, I would first make sure I burped this baby. I jacked the front wheels up a good foot off the ground and still had air pockets. You could hear it gurgle when you shut it off. Out of my desire to overkill this, I backed it over a bank that dropps about 8 feet in 20 distance. I got about another 3/4 gallon into it. Problem solved.
The fact that you keep adding fluid says it is either not full, boiling off, or leaking somewhere. First make sure it has no air pockets (I like the bank). It wouldn't hurt to check the
oil and make sure no water is getting into the
oil. Watch the exhaust to make sure no steam is coming out. You can try running it with the cap off to see if air bubbles is getting into the water from a compression leak, but I am getting ahead of myself. First step make sure there is no air pocket in the engine.