Gray water tank & guages

Well, we returned yesterday from a delightful five day adventure in our '07 2720SL at Lake Pleasant, northwest of Phoenix. We're not exactly newbees, for this was our 4th trip since acquiring the trailer in 9/06, but we've always had our suspicions about the gauges, especially the gray water gauge. After just two showers (that did not deplete the available hot water) and washing about a dozen plates and cups -- the gray water tank gauge registered "full". Maybe there were a few teeth brushings in there too.
Well, after emptying the tank, I decided to purposely fill it again and monitor the gauge as I emptied 6 gallon water containers into the shower basin. After 6 gallons the tank registered 1/3, after 14 gallons it registered 2/3, and at 24 gallons it registered "full". At 36 gallons it backed up into the shower basin and began to leak out from screened box beneath the trailer that protects the shower peetrap.
So my questions are: 1. Should I repair the gauge or let go of the expectation that the gauge will ever read accurately (kind of hate to pay for something that really doesn't serve me)? , 2. Is there an overflow valve for a full, gray water tank that that is supposed to vent through that screened black box or do I have something else that needs repair?
And I very much appreciate the expertise that I find in this forum. A few weeks back, I followed some well written instructions and photos and closed an annoying gap between the shells. I decided to do this myself rather than drive the TM several hundred miles to the Tucson dealership. Thanks for being there. Eric